$150 for a UV light?
If you're a DIY kind of person, you can get the UV LEDs off the internet (look for 400-360nm range - you can mix LEDs to get to the full spectrum) - each led should run about $1 per unit (if its more expensive its because its harder to make the low end of that spectrum - 250-350nm are the most expensive ones and can run to about 40-400 dollars per LED!).
Get a UK C4/D4 (cheapest), get the lamp out and build your circuit.
I think the D4 is better - more room to place more LEDs, the best layout is honeycomb.
I would recommend using NIMH rechargable batteries - you'll get hours of juice out of them - or if you have a lot of $$$ - get the monster battery (not rechargable - but 18AH!!!)
The only reason I wouldn't recommend to do it, is time and the risk of flashing that thing into someone's eyes, frying your buddy eyes under water is not a good idea.
(or above water for that matter)
BE WARNED, LOOKING DIRECTLY AT THOSE LEDS EVEN FOR A SHORT TIME CAN CAUSE PERMANENT DAMAGE TO YOUR EYES!!!!! DO NOT TRY THIS IF YOU DON'T KNOW HOW TO PROTECT YOURSELF AGAINST UV OR DON'T HAVE THE DISCIPLINE TO USE IT SAFETLY!!!
DO NOT FLASH IT ON SOMEONE ELSE, THE SCATTER CAN BOUNCE BACK OR WORSE, SHINE IN SOMEONE ELSE'S EYES!!!
UV is definatly not for kids, even those flouroscent blacklights can cause damage in prolonged exposures.
(how's that for a disclaimer?)