I have 3 nicrew lights that are all about 4 or 5 years old, they are still working well. The power to the lights is low voltage DC so I don't worry too much about them being waterproof. I have dropped a couple of them in the water and even though they were in the water they still lit. Took them out and dried them, still working, though I expect they are slowly corroding. If you have a saltwater tank I believe they would die soon after being dropped in the water. Mine are definitely not waterproof, and I expect if you used them long term over open water they might degrade due to corrosion. I keep mine over the glass covers to the tank.
Whoops. Mine are all just above the water, no glass or tight fitting lids on any of my tanks, since they're all second hand and ancient, with hoods and things.