From publisher blurb:
Underwater settings have been a large part of old-school gaming since the very beginning—underwater kingdoms, hidden coastal coves, monsters shambling out of the deeps. Creatures wondrous and strange and oft deadly, new magics and primal beings and ancient wisdom. These are the reasons adventurers descend into the salty depths of the sea. The darkest forest, the deepest cave, the largest dragon—none can match the sea for sheer scope of the unknown. Aquatic Adventures provides you with the tools to create your perfect underwater world.
There is a suggested world within the pages of the book, one that connects the various races of the sea by politics and ancient rivalries, but it's kept general enough to be portable to any world you are playing. The modularity of the world-building means you can add the bits you like and ignore the ones that don't work for you.
59 new magic items (including 16 legendary items)!
11 new spells!
8 new aquatic classes!
64 new underwater monsters!
New rules for adventuring beneath the water!
Lingomancy: an entire new kind of magic!
New Armor!
New Weapons!
Gods of the Deep!
Dug Too Deep: An adventure set in a haunted land beneath the waves!