I've used this type of strobe light for diving for years. The strobe works well when there are low light conditions. It's cheaper than the other strobes mentioned.

http://www.divebooty.co.uk/online_specials/1432.htm

It's my back-up torch as well in the rare event that my main torch fails.

I always use rechargable batteries, making sure that all the batteries, for this and all my other dive check is part of the preparation for going on a dive just as you would for a hike or other adventure.

Just my 2 euro's worth.