The RPG-7 has a piezo-electric fuze, and is very quick and very sensitive. The reason for this fuze is that shaped charges (which is what the RPG 7 warhead is) are very sensitive to stand-off distance. They work best when detonated some distance before hitting the target. (The long, needle-like nose of the warhead is empty -- the shaped charge is in the back of the case.)
If it hits water, it ought to go off on the surface. The detonation would take place a few inches above the surface, in fact.