"Moshe stretched out his hand over the sea, Hashem moved the sea with a strong east wind all the night, He turned the sea into damp land and the waters split."

Rashi explains the end of the verse as follows: "And the waters split", means that all the waters in the world split at that time!

This comment is based on the Medrash (Shmot Rabba, 21.6) which says: "And the waters split, the verse does not say the sea split, but the waters split. This means that all the waters in all the wells and the cisterns in all the world were split.

The medrash points to a change in the verse "Waters" and not "sea" which generates the comment. Rashi does not mention this point. 

Previously (verse 16) we learn: "…lift up your staff and stretch out you arm over the sea and split it; and the people shall come on dry land". Clearly the sea splits first and then there is dry land (or damp land). There is a further problem in our verse (21).

The Torah says that first there was dry land and then the sea split. It is possible for Rashi to argue (as he does on other occasions) that the order of the clauses in the verses is not indicative of the order in which things happened. If the verse says first dry land and then split, we can just reverse the order since the order of the clauses is not the necessary chronological order. This is similar to Rashi's use of the principle ein muqdam… there is no necessary chronological order preserved in the Torah. Here Rashi opts for the other possibility. The order of the phrases is correct but they refer to two different things. The first, "he turned the sea into dry land" describes what happened at that time. The second, "and the waters split" refers to another event, the splitting of the various waters of the world at that time.

It is important for Rashi to include this information, found in the Medrash, in his commentary in order to explain a theological dilemma. What was there about this miracle that was so significant that it produced the "and they believed in Hashem and in Moshe"

The answer for Rashi is that unlike the miracles in Egypt which were local and a milder aberration of nature, the miracle at Yam Suf changed nature itself, and created faith for the people of Yisrael.

Gut Shabbos,
Chaim Brovender