I once had a really cool dream where blizzard implemented a Maelstrom expansion that focused mainly on adding islands to the game.
Added cities included Undermine as a neutral city, an ancient troll temple to finally give the Darkspear Tribe a capitol, and Metrognome, a flying city belonging to the Gnomregan Exiles, similar to Dalaran stationed to spy on their rivals in Undermine.
Many islands had high cliffs and cave layouts which increased what initially appeared to be very small surface area, while other islands like Undermine were rather large both in apparent size, and surface area. There was also an instance that involved a ship that gets knocked off course and ends up sailing through the Maelstrom, with different monsters attacking along the way. The actual Maelstrom was a "locked" area to be added in a later patch as a raid.
After this dream, I decided to take a break from WoW for a while.
My point is, there's alot of Azeroth at sea that isn't necessarily under-the-sea. Furthermore, if Blizzard does implement an underwater dungeon/raid, I think they would supply us with the means to move relatively freely through them either through buffs, or vehicles.
As to what the next expansion will be, I'm leaning more towards the Emerald Dream. The portals are already there, and there's some nice plot advancement for it with the Scythe of Elune making another appearance, the Emerald Dragon Shrine, and of course Vordrassil.
Lastly, I'd like to emphasize that this entire post is speculation.