Since this is a "scuba" series of questions... Yes, I always carry a flashlight underwater. I carry the ScubaPro Novalight 720. It is always in my BCD. It is a little big and heavy, but it does a great job of illuminating all sorts of nooks.
When I am not diving, I always carry a Streamlight 73001 Nano Light Miniature Keychain LED Flashlight. It's very small and lives on my key ring. For it's size it does a great job.