This is what I got out of 2 pages so far:

-Natural occuring sea plant that [insert magical means here] filters water through lungs and eliminates the effect of decompression up to a certain depth.

-Weredolphins

-A LA +0 template for people who go through a magical transformation to become amphibious.

-A LA +0 aquan race close to humans (Sea Half-elves?)

-Zombie sharks and skeleton whales, oh my.

-Tritons are like the Persian empire at the height of its glory. Advanced civilization, enlightened, believe in the equal rights of all creatures but they're being attacked on all fronts and the rest of the world is unconcerned with what's happening underwater.

-Slave trade? Merfolk want slaves, sahuagin want live food and sacrifices...

-Water based human mercantile colonies. Massive coral reefs provide rare sea life for study, building material, and natural resources humans are trying to support. Pearl trade? Underwater based drugs (sea sponge snorting)?

-Merfolk are like the Mongol hordes. Tactical geniuses, masters of war, and ever expanding. They're at constant war with the equally expansive sahuagin, defend their homes from human expansion (a result of mercantilism), subjigate the sea-elves who make easy slaves, and have bitter but strained relationships with the tritons.

-Sahuagin are basically the same. Despite their numbers and raw power, they're too disorganized, shortsighted, and fight each other as much as their enemies to pose a world threat. Some sahuagin are more honorable (but still evil) and sell their services to the highest bidders much like bugbears.

-Underwater liches, F yeah.

-Special feats for combat underwater and more variety in underwater items (primitive depth charges? What kind of artillery would merfolk use?)

-Underwater class variants (sea bard, aquaman druid, casting spells underwater).

-Setting? Age of Sale/Enlightened period where underwater creatures are expanding as fast as humans on the surface or antediluvian earth where men thought the world was flat and feared the ocean (here be serpents)?

-Giant crabs. All things are better with giant crabs.