Anything that creates Ice could be handy...
"Remember, in an emergency, Wall of Ice may be used as a floatation device."
The reason I enjoyed reading Schätzing's 'Der Schwarm' was because it's full of fun and often little known facts about everything related to the (under)sea.
Yeah, his current novel (Limit) is even longeredit: Gah! I decided to buy the book... 1700 pages?!?
I'd be hesitant to say that it would *alienate* me. But to be honest, unless I knew the DM could do it well, I would pass. The problem with underwater D&D campaigns is that they tend to focus more upon what you can't do, as opposed to what you can do. There's so many classes, spells, powers, equipment, and skills that are pretty much useless underwater. As a PC, the thing I dislike the most is *not* being able to do stuff. Not to mention that three dimensional combat is a pain. Fun to do every once in awhile, sure, but horrible when its the norm. That's not to say I would be entirely opposed to playing in a underwater setting; its just that I don't think its a good fit for D&D. Something better for modern settings, like GURPS, would be more suitable.If your current DM approached you with the idea of starting a new campaign set primarily beneath the surface of the sea, what would be your first reaction? Suppose the “core races” were replaced with the likes of sea elves, locathah, and merfolk (or any race that that has a swim speed and the aquatic subtype). Would that be enough to alienate you?
Nah, actually I tend to think that Dragons are overdone. I think it's interesting you like to use hags, it would be cool to see what you'd do with them.I set my games on Oerth, the world of Greyhawk. Prior knowledge of the campaign setting is not required. I also tend to scale back on the use of dragons, while overpopulating the world with hags. Again, this is simply my personal signature in my games. Is that the killing blow that distances potential players?
The catch is, you're playing D&D. And for better or for worse, in recent years it has become a combat heavy game, and players have come to expect it. I, too, prefer role-play heavy campaigns, but we are clearly in the minority right now. I think an RP heavy campaign on top of a campaign setting which greatly restricts PCs is going to alienate most people who play D&D. I'm not saying its a no-sell, its just going to much more difficult to attract players. Of course, if you've already got a solid group of players behind you, this is not so much of an issue.My games tend to be role-play heavy and combat light. Rolling lots of dice tends to break my “willing suspension of disbelief”. Spending hours speaking in character as a room full of NPCs is my bread and butter. Again, I know this does not appeal to everyone.
I would expect large amounts of underwater cities, and creatures/cultures to communicate with. (rather than the large amount of "dead space" in our own oceans) I would expect conflicts with surface-dwellers. I could see underwater ecological catastrophes inspiring a group of individuals to explore the deep (or surface) for cures.With that in mind, what are your preferences, for such an adventure? What would you expect to see, in an undersea game? What would make the campaign memorable, enjoyable, and enduring? What would make you want to spend years exploring the realm of liquid space?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.