Whirlpool material & effect change + shooting fix.

Whirlpool material now matches lake material.
Whirlpool effect is to pull the boat's x towards the centre.
Whirlpool does damage over time (way too much?).
Shooting only affects layers "Animal" and "Boat". All obstacles are now
also on layer "Animal" because why not.
Overlay added for underwater (whirlpool) effect.