Here's some rules Jonathan Tweet and I cooked up.
If you don't have a swimming speed (or a freedom of movement effect), you suffer a -2 penalty to attacks and damage when underwater. Any slashing weapon, blunt weapon, claw attack, or tail attack also inflicts half damage. (Subtract the 2 points first, then take half of the rest and round down.)
Remember that a successful hit still inflicts at least 1 point of damage.
Even if you have a swimming speed, you inflict half damage with slashing or blunt weapons (but not claws) unless you have a freedom of movement effect.
