Dude, you gotta go big
DeepSea Power & Light - Underwater Lights
Sea Vision Underwater Lights
Seriously
Those lights get awfully hot. I wouldn't set one down on the deck until it cools down completely.
Before you spend that much money on a Xenon light, try some LEDs. You can gang a bunch of white LEDs together and get a pretty bright light. The LEDs will also be cooler and use much less power.
I've got a bunch of waterproof LEDs left over from other projects. I can put together a prototype to see how much light it will generate. I'll send you a PM when I'm ready to test.