Rashi on Exodus 14:21 says this:
AND THE WATERS WERE DIVIDED — all the waters in the world (Mekhilta d'Rabbi Yishmael 14:2:2).
Why did this happen? Are there reasons given?
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As explained in my comment, the sefer אורי וישעי explains this as follows:
נבקעו כל מים שבעולם כי בקרי"ס נעשה נס בעצם יסוד המים שהם החומר הגדול
However, I find the explanation found in Gevurot Hashem (42:1) the most suitable in this case.
The Midrash explains:
Immediately, Moses listened to God and went to split the sea, but when he went to divide the sea, it would not accept from him to split. The sea said to him, "I should split before you? I am greater than you! I was created on the third day, and you were created on the sixth day!" When Moses heard this, he went and said to the Holy One Blessed Be He, "The sea doesn't want to be split." What did the Holy One Blessed Be He do? He put His right hand on the the right hand of Moses, as it is said, "who caused [His glorious arm] to go at the right hand of Moses [dividing the waters before them to make Himself an eternal name]" (Isaiah 63:12) Immediately, it saw the Holy One Blessed Be He and fled, as was said, "the sea saw and fled" (Psalms 114:3). What did it see? It saw none other than the Holy One Blessed Be He who gave His right hand upon Moses, and it could not delay, but split immediately.
Later, the Midrash explains:
Immediately when Moses raised his hand against the sea, it was split, as was said, "The waters split" (Exodus 14:21). It does not say, the sea split, but rather the waters split to teach that all the waters that were in all the springs, and the well and in every place were split, as was said, "The waters were split." And so, at the time that they returned, all the waters returned, for such it says, "And the waters returned" (Exodus 14:28).
"But rather the waters split to teach that all the waters that were in all the springs, and the well and in every place were split". I think this is also what is explained in אורי וישעי.
Back to the Maharal in his Gevurot Hashem. My quick translation is as follows:
"ויבקעו המים" (שמות יד, כא). דרשו רז"ל (שמו"ר כא, ו) "ויבקע הים" לא נאמר, אלא "ויבקעו המים", כל מימות שבעולם נבקעו. וזה תבין ממה שבארנו לך, כי כאשר נבקעו המים אז היו ישראל מושלים לגמרי על הטבע, ונעשו קדושים נבדלים, והיו קדושים לה', כמו שהתבאר למעלה. ומפני זה היה קריעת ים סוף מצד המים בלבד לא מצד הים, כי ראוי שיהיו המים נבקעים ביום שביעי של פסח, ואז יצאו ישראל לגמרי להיותם מושלים על מדת המים, שהם חומרים בכללם, לכך כל המים בעולם נבקעו. ואילו לא נבקע רק הים, לא היה הממשלה לישראל רק על הים, ולא על כל מדת המים. אבל ישראל היו מושלים על כל המים, לכך כל המים שבעולם נבקעו.
וַיִּבָּקְע֖וּ הַמָּֽיִם- The waters were split (Shemos 14:21) "it is not said "And the sea shall be split up" but, "And the waters shall be split open", all the waters in the world are split up.[...] because when the water split then Israel was completely in control of nature, and became separate saints, and were holy to G-d".
Rabbi Yehoshua Hartman explains that overcoming the nature of the sea is overcoming the world of nature, which is a material world.
Similary, the Rabbeinu Bahya explains the kabbalistic view of this event:
the words “the waters split” (instead of the sea split) mean that the waters throughout the universe split at that time. The reason we must understand the words in that way is because had it been a local phenomenon only the Torah should have written ויבקעו מימיו, “its waters split.” There is another dimension to the splitting of the sea which will make you understand that it was essential that all the water in the world be split at that time and that a total change in the behaviour of natural law occur at that time.
See also this bonus from the Rabbeinu Bahya.
Similar to what @Shlomo wrote in the comments, Etz Yosef on Shemot Rabbah 21:6 wrote:
"מלמד שכל המים שהיו כו'. וזהו הירדן יסוב לאחור שכל המים נתבקעו אז. והיה זה כדי לפרסם הנס ושלא יחשוב שהיה זה מצד הטבע."
Translation: "Comes to teach that all the waters that were etc. And that's what it means that the Jordan moved back that all the waters split at the time. And this was to make the miracle known and that it should not be thought that this was a natural occurrence.1
1 As is thought by many today, that the splitting of the sea occurred because of the fall of the sea level.