For when between locathah, aquatic elves, kuo-toa, sahuagin, and merfolk you can't quite find the right underwater humanoid, here's skum, which originated from aboleth doing unholy things to humans.
Two aberration HD come with quite the interesting features. +8 strength is most notable, and the dexterity and constitution bonuses are interesting as well. -4 charisma sucks, but it's only charisma. Skum breathe both water and air, have a decent land speed and a good swimming speed, and even get a phletora of natural attacks (bite, claw, claw, rake, rake).
Finally, skum get a few skill bonuses when underwater.
I found myself considering both +0 and +1 LA, but I'll be going with +0 in the end. Skum are already losing a point of BAB, and half their stat bonuses could already be gotten by being a water orc. Also: aberration HD suck.