I have this setup:
Bright led flashlight with sliding switch that adjusts brightness. Accepts 2 batteries for hours of diving.
Underwater to 50M 1200 Lumen CREE XM L T6 LED Diving Flashlight Torch Dive Light | eBay
PS: Got tired of flooding my flashlights (not this one, just in general) and killing $10 batteries. So, went to plumbing section and got a toothpaste like looking tube of liquid silicone (the one that hardens when exposed to air). I add a tad on the threads a few hours before the dive for it to thicken. Worked every time well to 100+ feet. No leaks. Takes a tad more strength to unscrew but is well worth it, just doesn't leak. Now one less thing to worry about while diving.
PPS: I usually take a new flashlight and disassemble all external parts that come off. Apply some silicone before putting together. Places like where the lens is mounted, where the head is mounted... etc. Makes very, very waterproof. Even Chinese flashlights that might have tiny flaws in clearance of parts act like more expensive flashlights with the same dependency.
PPPS: Forgot to say, on ebay there are flashlights of this type and size but they have more LEDs in them for higher lumens, don't get them, their beam is not as tight as a single LED one.
2 batteries 18650 from Panasonic. Protected from low voltages. Even though says 3400Mah, it is the TRUE mah, not like 5000 Cheese mah...
NCR18650B 3 7V Li ion Protected PCM Rechargeable Battery 3400mAh for Panasonic | eBay
PS: These are great. Before I was buying other cheaper brands but this particular one outshines them all. Seriously, it packs WAY MORE power than any other battery offers, and I think I tried a lot of them before I came to this conclusion. These are worth buying. Not sure about this specific seller, sure there are fakes ones out there. But in general, on ebay search for these ones.
Greatest charger ever. Charges pretty much anything that fits.
Nitecore D4 LCD Intelligent Circuitry Global Insurance 18650 26650 14500 Charger | eBay
PS: Throw away other chargers, they can harm your battery. They are dumb chargers. Get this charger once, job is done!
So price-wise:
~$30 flashlight
~$20 2 x batteries
~$20 2 x batteries (extra set)
~$20 charger (can charge all 4 batteries at once)
~$5 silicone tube from home depot or something (make sure it is the one that can be re-sealed).
~$1 rust proof key ring
~$5 butterfly marine bolt (so you can attach the flashlight to yourself)
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~$100 for a great flashlight.