The stubborn sea
During the holiday of Pesach (as well as every other holiday), we recite Hallel during the morning prayers. It consists of chapters 113 to 118 from Psalms. Chapter 114 describes how when the Jews left Egypt, nature was entirely subservient to them. Nothing stood in their way. Most pronounced was the miracle of the splitting of the sea. On the seventh day of Pesach, we commemorate this event with the Torah Reading being the Song at Sea that the Jews recited after this miracle. In Psalms the sea is described as “running away” from the Jews, meaning that it split in two, after seeing something. What did it see that made it split? Some say that it was Moshe. Others says that it was the coffin of Yosef. A very strange opinion is that the sea “saw” the teaching of the Academy of Rabbi Yishmael. What does this mean?
At the time that the Jews were at the sea, there was a glaring contradiction. Hashem was going to split the sea for them, to rescue them from the Egyptians, but why? The Jews were until recently idol worshippers, just like their pursuers. What made them special, to have this miracle performed for them? The Egyptians were just as deserving to have their goals met. Therefore, the sea didn’t want to split.
There’s a halacha regarding a Jew who willingly sold himself to idol worshippers. He becomes a slave to an idolatrous Temple, and has to perform all the various tasks to help the service of the idol. Since he put himself in this situation, maybe we should leave him be? The Torah tells us that no, we must have mercy on him and get him out of that lowly life. Even though he put himself in that position, we can’t abandon him. It happens to be that the author of this teaching is non-other than Rabbi Yishmael. It was this teaching that the sea “saw”, and was therefore forced to split. Since this halacha was said regarding Jews, the sea needed to split to save them from their idol worshipping lifestyle. Staying in Egypt would have meant maintaining their idolatry. Leaving Egypt was the beginning of their journey to Mount Sinai, to receive the Torah and fully abandon their idolatrous past. The claim that the Egyptians are no worse no longer applied, as the whole point was to save the Jews from idolatry. Therefore, the sea split.
A further explanation given for what the sea saw was the massive wealth of the Jews. What does this mean? When Avraham was first told that he would be a great nation, Hashem informed him that his children would be slaves in a land not their own for four hundred years. In reality, the Jews left in less time than that, staying in Egypt only two-hundred and ten years. As a result, when the Jews arrived at the sea, it didn’t want to split. It wasn’t yet their time to leave Egypt. However, the Jews didn’t arrive at the sea emptyhanded. They bore incredible wealth, which they had received from the Egyptians. This wealth was in fact the traditional payment that is given after a Jewish servant goes free. This payment is given only when the Jewish servant completed their full period of servitude. As such, when the sea saw that they had this incredible wealth, it concluded they had in fact completed their servitude. It thus split for them, allowing them to escape to freedom.
These two explanations actually work together really nicely. We could have asked on the first explanation, how did the sea know that the Jewish people at that time had the status of Jews? It was before they had received the Torah. Who says they deserved to be rescued from idol worship, like a Jew is? The answer is that it saw their incredible wealth. This wealth was the payment for their servitude in Egypt. The payment to a servant who completes their servitude is only given to a Jewish servant. Since they had received such a payment, it proved that they already had the status of Jews at that time. Therefore, it split for them for both reasons: because they had completed their servitude, and because they deserved to be rescued from idol worship.
Chag Sameach and Good Shabbos!
 Based on Shir Maon (understood to be written by the Chasam Sofer‘s grandson Rav Shimon Sofer), brought in Toras Moshe IV, end of parshas Tzav under LePesach, s.v. שמעתי מבני, quoting his son Rav Avraham [Sofer]
 This verse is referring to the ים סוף, often translated as the Red Sea, but more correctly as the Reed Sea
 Psalms 114:3
 Exodus 15:1-18. This is because the sea split on the Jews’ seventh day of their journey
 Megillah 31a; Shulchan Aruch Orach Chaim 490:5
 Shemos Rabbah 21:8. Others there suggest either the merit of Avraham or Yaakov
 Literally: the bones of Yosef
 Mechilta D’Rabbi Yishmael 14:15; Bereishis Rabbah 87:8; Midrash Tehillim § 114. In Sotah 37a (with Mahasha ad. loc) it’s said to be either the tribe of Binyomin or Nachshon Ben Aminadav’s act of jumping in the sea that caused it
 This Midrash is brought by the Chasam Sofer in Toras Moshe I, end of parshas Tzav under LeShevii shel Pesach s.v. ויבקעו המים במדרש רבה and הים ראה וינס, and Derashos II p. 269d s.v. ראיתי, the latter citing Zera Yaakov (Katan). The source for this Midrash is unclear. One sefer I found claimed the earliest source they found for it is Tzafenas Paneach Chadash § Kayin VeHevel UKrias Yam Suf (published 1694), in the name of a certain sage
 The Shir Maon uses the phrase הללו עובדי עבודה זרה והללו עובדי עבודה זרה, which I heave heard quoted in many contexts. This phrase does not seem to have a source in Chazal. The closest is the Zohar II p. 170b, although that’s the Aramaic version of this phrase, and it adds both also were guilty of illicit relations and murder. The closest in Hebrew is Shemos Rabbah 21:7, but it only says said that the Jews until recently were idol worshippers, with no mention of the Egyptians. See also Shir HaShirim Rabbah 2:1, 8:8 that the ministers of the nations in the future will claim against the Jews אלו עבדו עבודת כוכבים ואלו עבדו עבודת כוכבים, and Midrash Tehillim 1:18, 15:3 that the nations in the future will claim אלו עובדי עבודה זרה ואלו עובדי עבודה זרה
 The Shir Maon says that it was the sea that made this claim, but Zohar loc. cit. says it was the Angel appointed over Egypt, and Shemos Rabbah loc. cit. says it was the Satan
 Leviticus 25:48
 Rashi to Kiddushin 20b s.v. הא אהדריה קרא
 Kiddushin loc. cit.
 I couldn’t find any early source for this explanation. One source that predates the Shir Maon is the Yismach Moshe Bo 1:9. A lot of other chassidishe sources bring it as well
 Genesis 15:13
 Rashi ad. loc. and to Genesis 42:2 and Exodus 12:40, based off of Megillah 9a. See also Bereishis Rabbah 91:2
 הענקה, as seen in Deuteronomy 15:14. This payment is a positive mitzvah (Sefer HaMitzvos Aseh #196; Sefer HaChinuch #482; Semag Asin #84)
 Rashi to Deuteronomy 15:15, quoting Sifrei Devarim § 120
 Kiddushin 16b
 Cf. http://parshaponders.com/pesach-2-5778, which brings the Chasam Sofer’s opinion that the Jews did not yet have their Jewish status, and as such has an interesting explanation for why the sea split the way it did