ExploderWizard
Hero
Underwater adventures can be a lot of fun. As a DM it also helps you to keep the pressure on the PC's 

Why did I choose to set a campaign not only in the World of Greyhawk, but also primarily underwater? <snip 495 words that don't explain why the campaign was underwater>
Almost on a whim, I decided to set the game underwater. I had started keeping saltwater aquariums and my campaigns already had a reputation for being somewhat unorthodox, so it seemed a natural fit; the perfect amalgamation of my interests in writing, D&D, and marine aquaria. <snip 140 more words; end of post.>
You mention "hydrodaemons" -- underwater demons are a great new idea too. What kind of flavor and abilities do they have?
Are you perchance anywhere close by?
My LFS, Aquamain's, just changed hands. I briefly considered buying him out but the economy just doesn't seem right. Perhaps in 2-3 years I will open an aquarium store...hrmm... Finland?That's pretty close, but too far for a gaming commute. I work at Fintastic aquarium shop in Charlotte (just moved here from Seattle about 2 weeks ago)
As I have posted elsewhere, I like to interject a bit of reality into my fantasy; my blinogo (sea goblins) were inspired by goblin sharks, iblishi (sea kobolds) by blue-dot stingrays, koolocanth (sea bugbears by coelacanth, and krana (sea orcs) by black chimaera.Had not thought about looking into a tank for monster and dungeon inspiration; what a great idea! Huge anemones and tunnels dug by enormous yellowhead jawfish....
<snip>
3) Anything that eats its live prey whole...sloooooowly...creeps people out. Most people would think constrictor snake first, but they don't do the live part. Cone Snails, OTOH, use a harpoon-like radula to inject a nonlethal paralytic poison into small fish (and other prey) and eat them whole. I used a Giant sized one of these in a campaign once...the hate of the thing was almost visceral.
Elsewhere, I had even posted about an aquatic Monk variant that, like nudibranchs, had the class ability to consume anemones and incorporate the nematocysts into their bodies...
In my games, I had the Prophet A'nan, whose followers were anemoids, small humanoid anemones.