Sea Hunter

Combat Expertise, merfolk.

When you make a successful melee attack against a swimming target, as a free action you can attempt to knock the target off balance. Treat this as a trip combat maneuver. If you succeed, the target is considered off balance until it recovers its balance, usually by making a Swim check on its turn. This feat has no effect on creatures with a swim speed, those using magic such as freedom of movement, and creatures that can't be tripped.