Diving in 3D will be beautiful, I hope... I don't know why, but the vast sea of Hoenn is my favourite place we've seen so far in the 6 generations of Pokémon...
One of the things I've been wondering about diving is if, because we're playing in 3 dimensions now, is if we'll be able to dive to different depths. Like, that long undersea trench between Mossdeep and Sootopolis, it'd be cool to go to two or three different levels of that, with different items being found in different elevations and the scenery changing.