Your blows knock swimming opponents off balance.
Benefit: 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 (see Table: Combat Adjustments Underwater) 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.
Pathfinder Roleplaying Game Advanced Race Guide © 2012, Paizo Publishing, LLC; Authors: Dennis Baker, Jesse Benner, Benjamin Bruck, Jason Bulmahn, Adam Daigle, Jim Groves, Tim Hitchcock, Hal MacLean, Jason Nelson, Stephen Radney-MacFarland, Owen K.C. Stephens, Todd Stewart, and Russ Taylor.