It would be better if the splash effect occurs when the chopper hits the water, not when it already in the water. Actually, same thing with ground explosions, choppers in Arma explode when they are on a ground for a while, not when they hit it.
But that's nitpicking, very nice effect
Btw, somewhere in your script thereare these 2 lines :
_x = getpos _gl select 0
_y = getpos _gl select 1
_x and _y are defined before the gamelogic (_gl) is created
I removed these 2 lines and script still works fine.