That's one reason I've used the World of Greyhawk for my campaign setting of choice, for all of my online games (save for one set in Hades) since 1995. I can utilize the maps, history, climate, politics, religion, and sites of interest as a springboard. Mind you, not a lot has been written about the undersea realms, which is just as I like it.
Granted the Sinking Isle, Turucambi Reef, and Jungle of Lost ships are all canon Greyhawk references - but I put my own unique spin on them all.