In the 2007 Backpacker Gear Guide, there an ad for Surefire hand lights showing some caver in a waist deep water passage using a headlamp and what is presumably one of their flashlights. Just one little problem: their lights cost between $100 and $300 each, and the caver is apparently using one of the big $300 lights to illuminate the passage. I could just see the guy dropping the flashlight and never seeing it again. I can buy a lot of less expensive but waterproof flashlights for the price of that light, and keep a spare or two in my pack.
Issue: what's good enough for a second or third source of light and what's overkill? My backups are almost always Mini-Mags with the NiteIze LED conversion--New Mexico has very few caves that wet, though.