As you are going to the open sea, waves starting to appear. As far as i'v seen the craft is moved by theese waves between -4m an +4m ASL.
If you are going with boat propellers, everytime the craft is going to +1m ASL an above the propellers stop working and the craft slowes down until the waves ends and the craft is back to 0m ASL and below.
Vice versa, if you place jet engines on a boat it will work fine as long as the waves keep you above 0m ASL. With negative ASL the game seems to pretend you are under water, but visual you are between to waves.
In short, the optical effect of waves moves the water line up and down, but technically it seems to be nailed to 0m ASL.