Because divers experience weightlessness, tiny is not so much a requirement underwater. Although many of our tactical lights are rated for waterproofness, you might still want to go with a "dive light".
The Ikelite PCa holds 6 AA batteries (they also have a smaller 4 AA version) and is very bright. I haven't figured out what kind of LED they are using but theyare advertising as a "5 watt" LED. Might be the brightest stock AA underwater diving flashlight out there? I also have a Princeton Tec Torrent LED, but it is larger and dimmer. However, it appears to be ripe for a LED swap to a Seoul P4. That is my project for this afternoon. . .