Purchase a dive flashlight used for scuba diving. I've used an Underwater Kinetics light for years. The Mini Q40 MK2 LED is a basic underwater flashlight that operates for several hours off 4AA batteries. If you clean the flashlight with fresh water after each use and remember to take the batteries out when storing for long periods, it will last for many,many years. My wife's light (an earlier version) is almost 12 years old and going strong. I replaced mine only because I lost it at The Indians a few years ago. Also, make sure the lens cap is sealed tightly before it goes underwater and you are good to go. For a light that will last several years expect to spend at least $50. The Q40 runs about $60 at scuba.com.