Apparently, just like in the video, the Chacha will follow you underwater, and has a special "dance" attack. In the video, the "dance" appeared to summon a large monster, but we aren't sure if that's what it really does in-game.
As much as we enjoy looking at this Monster Hunter stuff, we must still note that the game hasn't been officially dated for North America yet. Please, Capcom? And if it wouldn't be too much trouble, we'd like it in bundle form as well. Thanks!