I recently purchased a PDF of Shadowsea from ANTIMATTER GAMES, a rather interesting game that uses the Song of Blades ruleset but puts it into a subterranean world. Having perused it for a bit I must admit its well worth the money!
Theres some interesting forces ranging from treasure seekers, to templars, lost civilisations and my personal favourites, the Dark Mariners!
With concept sketches like this its easy to evoke a flooded realm full of deadly creatures:
Shadowsea Concept Sketch
I shall have to see what I can come up with!
All the best!