Most of the famous pshatim on "it is as hard to match them as to split the sea" are boring. So here are the ones I like. The ones I don't attribute are my own.
Note to speakers: As Eli points out in the comments, it is hard to deny the pashtus that this expression refers to second marriages, not first marriages. However, as I respond there, hard, but not impossible. Many many meforshim apply it to first marriages, as indicated by its use in Midrashim. In any case, you should be prepared in case someone comes over later to correct you.
1. Even though Kri'as Yam Suf was preordained and inevitable- the Red Sea was created on the condition that it split for Bnei Yisrael, as taught in Breishis Rabba 5:5, אמר ר' יוחנן תנאין התנה הקב"ה עם הים שיהא נקרע לפני ישראל הדא הוא דכתיב (שמות יד) וישב הים לאיתנו לתנאו שהתנה עמו - the Ribono shel Olam wanted the Jews to cause it to happen with their tefillos and Emuna and mesiras nefesh. This is hard for the Ribono shel Olam because Hakol bidei Shamayim chutz mi'yiras shamayim. Same with shidduch; it's preordained, but the Ribono shel Olam wants hishtadlus and courage and tefilla on your part to make it happen. Part of the reason for this is that we need to learn that we can overcome our past. The Ribono shel Olam helps us, and directs us, but we ourselves need to develop the strength of character and faith to make a break with the past.
Please note, too, that at the Yam Suf, Bnei Yisrael were surrounded by temples of Avoda Zara- 14:2, לפני פי החירות בין מגדול ובין הים לפני בעל צפון נכחו תחנו על הים see Rashi there. (Also, note that besides Baal Tzefon, Chiros may refer to the falcon-headed Horus, one of the indefatigably libidinous Egyptian Gods, as hinted in Maseches Kalla Rabbasi 7:4, כיוצא בדבר אתה אומר מה זכו החירות אלא זונות היו וכשראו את ישראל הפכו את פניהם לפיכך נקראו חירות.) Their fear of the Egyptians might have been aggravated by the proximity of the Gods of Egypt whom they still feared as independently powerful. It was because of this fear of the past, this fear of the Egyptian Gods, that it was said about the Bnei Yisrael ve'halalu ovdei Avoda Zara. At that very moment they showed their superstitious belief that the Gods of Egypt might fight the Ribono shel Olam. The only way Bnei Yisrael liberated themselves from these indoctrinated superstitions, the only way they could break the crippling chains of the past and embrace their bright future was by leaping into the sea in a physical demonstration of single minded faith and trust in the Ribono shel Olam.
2. Kri'as Yam Suf involved midas hadin for the Egyptians and midas harachamim for the Jews. To manifest the attribute of strict justice and the attribute of loving mercy in one event cannot be done through a shliach. Only the Ribono shel Olam alone can do this, as was the case in Mitzrayim during Makkas Bechoros. In Mitzrayim, though, they had the zechus of doing the korban pesach, which was mesiras nefesh, and that made it easier to save Klal Yisrael, because mesiras nefesh is equivalent to netillas neshama. In married life we need to learn to judge and discipline ourselves with strict midas hadin and our spouses with loving rachamim.
3. The chidush and the symbolism was that after the phenomenal pirud, the sea rejoined as if nothing had happened. The most important rule of marriage is that inevitably, there will be strong differences of opinion. When that passes, it should be as if nothing had happened. While Klal Yisrael walked through, the sea became a beautiful path with sides of solid windows made of water, when the Mitzrim came after them, it changed into a churning maelstrom of chaos and destruction. Five minutes later, you could go out in a row boat and you wouldn't notice that anything had happened. (See studies by John Gottman, respected marriage counselor and erstwhile Lubavitcher.)
4. Chazal use the expression "Kashe K'krias Yam suf in two contexts: in Sota 2a on Shidduchim, and in Pesachim 118a on Parnassa. (I've heard shadchanim say that for them to make a living is twice as hard as Kri'as Yam Suf, because both Gemaros apply to them.)
When Hashem told Avraham about Yetzias Mitzrayim, it became a foregone conclusion, and the Yam should have split immediately when the moment came. It was a tnai with the ma'aseh breishis that was applied to the children of Avraham Avinu. The problem was that the Sar shel Yam complained "I would be happy to split for the sake of the children of Avraham Avinu, but who are these people?? They look like Egyptians with Peyos. They're the same ovdei avoda zara as the Egyptians. Why should I split for these people?"
When a person davens for parnassa on Rosh Hashanna and Yom Kippur, he is like an angel, his thoughts are lofty and pure, he is a worthy recipient of the brachos the Ribono shel Olam showers down upon him. A few months later, the bracha of parnassa is supposed to be fulfilled, and the Sar of Parnassa looks down at the person, and looks at his delivery order for the person who got the bracha, and he says "Hey, this is not the same person. Who is this guy, and why should he get the bracha of parnassa? I must have the wrong address."
When a person is going out on dates, he opens the door for the girl, he listens attentively to all her naarishkeit, he notices her tznius and her refined deportment, he is careful to make sure she is comfortable, and he is the perfect suitor. They get married, a few months pass, and they get used to each other and start taking each other for granted, and he's not opening doors any more. The sound of her voice either puts him to sleep or doesn't let him sleep. It's not the same.
Kashe zivugam kikrias Yam Suf; when the promise was made, everything was perfect. But when it comes time to fulfill the promises, they start to look like entirely different people. The chochma, and the challenge, is that when you need the parnassa, you look like you did when you made those promises on the Yamim Nora'im; the challenge of Krias Yam Suf is that the entire year we should look like the descendants of Avrohom Avinu, to whom the promise was made; and the challenge of a good marriage is to make sure that you look and act like the suitor who made the promises to respect your wife yoser mei'gufo even when you're long married.
5. A pshat submitted by lu: Unlike a solid that is split, a divided liquid requires constant effort to maintain that state; no less energy is required to sustain it than was invested in creating it, and that investiture of energy will have to be constant and ongoing. The symbolism is although marriage creates the new entity of a married couple, perpetuating that state requires constant attention and effort. It can never, ever be taken for granted. (This teretz, though very good, was kind of too straightforward and earnest for this post, but then I realized it shtims with the Rogotchover in Tzafnas Pa'anei'ach Hilchos Trumos page 20, and Reb Elchanan in Kovetz Shiurim Kiddushin #61.)
6. A pshat submitted by the Pelleh Din, The Hermit and Rav Hakollel of Manchester והגלילות:
Kriyas Yam Suf is מוציא אסירים בכושרות - בבכי ובשירות
People tend to think that by getting married they are losing some of their freedom, but from the analogy to קריאת ים סוף we see it's really a freedom. Accepting an עול granted - so was the יציאת מצרים and acceptance of the עול תורה. As Chazal say, אין לך בן חורין אלא מי שעוסק בתורה.
7. I saw this from Rabbi Frand, and, as usual, it is excellent.
When It Comes To Finding a Mate -- Providence Will Handle It
The Medrash [Bereishis Rabbah 60] says that there were four people in history who made improper requests. Three of these people were fortunate, in that even though their request was articulated improperly, G-d answered them kindly. The fourth person received a terrible response. These four people were Eliezer, Kalev, Shaul, and Yiftach.Eliezer said "And it will be the girl that says 'Not only will I give you to drink, but I will give your camels to drink' will be the girl who will marry my master's son." [Bereshis 24:14] The Medrash says that this was not an appropriate way to ask. What if a maidservant had come out and met all of Eliezer's conditions? Fortunately, G-d sent Rivkah.Kalev promised to give his daughter in marriage to the person who would capture the city of Kiryat Sefer [Yehoshua 15:16]. Kalev did not know what kind of person would conquer the city. What if a slave would have conquered the city? Fortunately, Osniel ben Kenaz conquered the city.Shaul promised his daughter to the person who would kill Goliath [Shmuel I 17:25]. Again this was a rash promise, but again G-d was kind and Dovid killed Goliath.However, there was one man whose improper request resulted in tragedy. Yiftach was praying for success in battle against Ammon. Yiftach said to G-d, "If You deliver Ammon into my hands, the first thing that comes out of my house to greet me, I will offer as a sacrifice..." [Shoftim 11:31] Unfortunately, his daughter was the first to greet him. [According to the commentaries, Yiftach either sacrificed his daughter or sent her away for the rest of her life. Either way, this was a punishment for Yiftach's improper request.]The question can be asked: Eliezer, Kalev, and Shaul also asked improperly. Why were they fortunate and why was Yiftach struck with tragedy?The Beis Av offers a beautiful insight. The Beis Av says that the common denominator of Eliezer, Kalev, and Shaul was that their requests all involved Shidduchim, marriages. When it comes to finding a proper mate, that is one area regarding which a person can legitimately say "G-d -- take care of me!" When it comes to finding a person's life mate, the rule is that this comes from the Hand of G-d [Yalkut Shimoni 117]. Regarding Shidduchim one is allowed to play with Providence and rely on G-d's intervention.Yiftach's pledge had nothing to do with a shidduch. When one utters an irresponsible pledge like that, anything can happen.I once heard the following from Rav Simcha Zissel Brody, who heard it from the Chazzon Ish. The Chazzon Ish commented, there are two things that are "as difficult as splitting of the Red Sea" -- a person's livelihood and finding a mate. We all know that a person's livelihood is difficult because Adam received a curse that it should be difficult to bring forth bread from the earth. But, the Chazzon Ish asked, why should finding a mate be so hard?The Chazzon Ish explained that unfortunately, we live in a time in which we usually do not see the Hand of G-d. However, there is one area where everyone admits that we often can still see it. G-d takes a special interest, and plays a special role in Shidduchim.We all have stories that we feel were 'basherte'. I will just say over my favorite shidduch story. This is a story of a young man who went out looking for a shidduch for a very long time with no success. He went out with his seventy ninth girl. It was a terrible date. He came back upset and broken. He decided to go buy something to eat. He walked into a Kosher hamburger place in Borough Park, sat down at a table and ordered a burger. A woman came up to him and asked, "Are you married?" He responded "No". She said, "I have a shidduch for you." This was a woman completely out of the blue, who had never met him before! Yet he and the girl suggested by that woman are now living happily ever after.This is an example where we see Hashgocha (Divine Providence) -- the Hand of G-d. Here a person can take the liberty and say "Master of the World, take care of me."
I have to add, however, that Rabbi Frand's quote from the Chazon Ish needs some improvement, and, as a contemporary from our days as students in Ner Israel, I'm confident that when he said it he said it originally he made this clear. The Chazon Ish's words don't explain the language of the Gemara that וקשין לזווגן כקריעת ים סוף, because קשין indicates difficulty, and how does this explain difficulty? The answer is that nothing is קשה for the Ribono shel Olam. The word Kashe in this context means that the Ribono shel Olam needs to overcome the preference to leave the world to Derech Hateva, to the ostensibly natural flow of cause and effect. When Hashem engages in obvious suspension of nature, that is called Kashe, difficult. Shiduchim are an overt intercession, a clear suspension of the randomness of Teva.