Well, the Servants are humans who have been "upgraded" with parts like gills, night vision and other fun stuff. I'd look to the Creature powers for ideas. The Fomor themselves are aquatic, related to the Sidhe and have used entropy and water magic. A Fomor Lord makes an interesting appearance in the short story Bombshells, if you're looking for an official source of information. But we really don't know much about them or their motivations.
