Implement swimming

[RailKill]

A game mechanic where if the player is in the water, moving the hand fast (high power) will paddle the player. To move forward for example, the player must slowly move the hand forward (low power), then quickly move the hand backwards (high power) so that it kind of drags and paddles the player forward.

Power in this case is the velocity of the hand, which should already be recorded in player.gd.