I'm with Eagle4; water needs something. It's such a shame to see an ocean and be able to do nothing at all with it, or have no reason to explore its depths. Just a waste of area. I know Notch wanted to do water stuff after a scuba trip he did, but now that he's not in charge I hope that ideas not dead...
Oh, this. So much.
At the moment, oceans or even lakes are just vast swathes of map with dirt, sand, and stone under a massive sheet of water. There's the occasional ravine here and there, but otherwise it's just too boring. The squids bring a little life to it, but... nah, it's boring. Underwater ravines are a chore to navigate too, you might as well dig elsewhere.
What about shipwrecks with sunken treasure? Underwater villages à la Atlantis? Coral reefs? Fish? Jellyfish, if you want enemy mobs that wouldn't be too hard to handle (this was the reason why sharks were dropped)? Heck, even crops that could only grow underwater would be an improvement.
When I find an Ocean biome, I sort of think "This is the edge of the map". Yes, I know there's land beyond it, I've even built a bridge across an ocean once, but usually there's no reason to ever explore the sea. An exception would be the rare occasion when Strongholds generate under the sea, or if you start on a very small island.
I hope the next update is sea-focused. There's a lot of potential with it, but so far, it's very unused. In Tekkit, I sometimes make offshore installations, but it's less rewarding than digging on land. I, like many others, dream about being able to build proper, custom boats and larger ships, but that would be hard if not impossible to do in the Minecraft engine.