How about a monster feat for pathfinder? No love for the swimming monsters?
Flyby Attack (Monster)
This creature can make an attack before and after it moves while swimming and does so gracefully.
Prerequisite: swim speed.
Benefit: When swimming the creature can take a move action and another standard action at any point during the move. The creature cannot take a second move action during a round when it makes a flyby attack. The creature does not provoke an attack of opportunity from the target of the attack.