Ha ha! Not quite that big Dove. But they're very very dark colours and so suddenly appear at the surface before you see them. Certainly I think they' could have my thumb off if I got too close*.
http://instagram.com/p/jT1d52o0H6/ (tried to take a video of them but I'm not sure it worked terribly well).
*I know logically they wouldn't but that doesn't stop my keeping my distance!!