I remember many years back on PBS someone had a documentary about a theory of how the red sea was parted. The area that Moses and the Jews were thought to have crossed at is actually very shallow. (easily verified by a geology study) Compbined with very high winds and a drought, portions of the sea actually move away for a length large enough (and shallow enough) for people and gear to move across it.

The scientists tested their theory in a lab using models and wind tunnels. It worked for the demonstrations and they had enough real scientific base to think that it was possible to actually have the red sea parted.

Its possible that God sent the winds and the drought and opened up the sea just enough for the jews and Moses to cross. Then shutting it down just in time to stop the pursuing army.

Basically we don't know.