I played and unlocked all areas in the first Endless Ocean as a rental. I found it neat, but not really worth a purchase. Then Blue World came out and blew me the heck away.
Basically, if "yee pretty dolphins!" and "yee pretty fishies!" are valid reasons for you to enjoy a game, as it is with me, this game is awesome. I don't know how much fun it might be for the non-yee-pretty-fishies crowd, so if you're not sure I'd give it a rental.
If you've played the first Endless Ocean and enjoyed it, well! The sequel is about ten times more awesome, with a better plot (such as it is), a lot more variety of stuff to do, and many more locations and pretty fishies to look at, including an Amazon River tributary with freshwater species. Also, some animals are actually dangerous this time around, and have various ways of either calming or avoiding them. Of course, it can be a bit narmy since nothing ever tries to actually eat you and the worst that happens is your air meter goes down some, but for the early stages of the game it can provide some excitement. Fundamentally, though, the game is still a relaxing one.
Since the game is nearly completely centred around swimming about looking at pretty things, the graphics are very nice, at least to my non-HD eyes. One unlocks four basic "mission" types as the story progresses: giving guided tours, taking photographs for magazines, treasure hunting, and giving dolphin shows with the assistance of wild dolphins and small whales that befriend you. There's also an aquarium to take care of where you can show off all the pretty animals you've met, and quests and special requests noted in your handy-dandy notebook. Any of your dolphin partners can come diving with you, and there's also three human characters who will join up with your diving service: each partner, human or cetacean, has their own specialty.
This is a very long game. The story isn't, but once it ends almost everywhere is open for exploration, and there's a donkey-load of crap to do, as well as titles/achievements you can earn for doing everything from befriending dolphins to doing set numbers of missions to getting attacked often enough.
Excellent game if it's your thing, and much improved over the first.