I got Endless Ocean for the Wii today.
I expected a quick swim around and look at fish, then put it down again game
It's stunning. I spent hours just swimming. Then I got a dolphin and discovered caves and an abyss and oh my. You can even load your own MP3s into it so you can listen to them while diving, though the game music is atmosperic enough you don't really have to.
Our SD card is broken anyway, so I couldn't try the MP3thing unfortunately.
You can play with friends over WIFI too (alas another thing I haven't tried). If anyone does get it, drop me your friends code so we can have a swim.
Buy this game. It sounds boring but it is worth having, especially if you already enjoy SCUBA.