For what its worth, I swear by the Fenix E01. Its tiny, light weight, burns for 21 hours on a single AAA battery, bomb proof and always works, nothing to go wrong, will happily cope with a soaking/ short dives and gives enough light to find your way out. I typically have it on neck string inside my undersuit (with a small knife and whistle if Im feeling consciencous). Have a snoopy on helmet for it but you can cave fairly well with it in your mouth. I faked light failure once and did just this cause I wasnt inclined to chat to a particular person on the trip
They also cost just 8 to 12 US Dollars. Potholer put me onto them a few years ago, which is recommendation enough for me.