Originally posted by Megahurts:

A ship will only move if there's someone on it or it's getting pushed by player, a creature or wave action if it's in shallow water.

I believe what the OP means is they go as fast as possible onto land, but even when you jump off it the "anchor" doesn't actually kick in until it is in the shallows and will move backwards until it is. I can say with 100% confidence i've noticed this myself too.