I heard from one of my rebbeim that the inifinite God has an infinite amount of ways of performing any given task. With that in mind, it is interesting to note that twice in the Tanach, God splits the sea: once when the Children of Israel leave Egypt and once when they enter the Land of Israel. (The second instance wasn't a sea, but the Jordan River, but I'm sure you understand what I'm saying.) Why did God choose to perform this particular miracle twice?
Avot Derav Nattan 33:2 implies that splitting the sea was one of the demands that B'nai Yisra'el asked of Moshe when they were at the sea.
Moshe repeatedly said to them - "Arise and cross". Each time , they demanded some condition for crossing. One of them was that the sea should be divided into multiple parts.
It seems, then, that God was responding to their demands.
Both times it happened after Bney Yisroel circumcised - first in Egypt before the Exodus and the second before entering the Holy Land (the didn't circumcise in the wilderness).
Passing thru the split sea symbolized immersing in Mikve, which is the second part of converting to Judaism for the whole nation. This represents (Kabbalicly) a rebirth or starting a new life.
Source - כתבי אריז"ל cited in בית גנזי על התורה by R' Rafoel Moshe Luria Z"L