as a cave diver, I will argue 100% that you need at least two lights when cavern and night diving.
You can do it cheap, or you can do it safe, take your pick.
My current preference is the following
You need/want one of these. This is a brilliant hand mount for small lights.
@stuartv posted a review a week or so ago with some of his lights. We have a difference of opinion on the lights he uses, but the glove is the same. This is key for not dropping lights as well as having your hand to grab stuff if you need to.
$30, write it down. When you order this, get a pair of small bolt snaps and ask them to throw some cave line in for you.
Cave Adventurers - OxyCheq Raider Light Sock (Raider III) - Marianna, Florida USA - Never Undersold!
Second is the light. Lots of options here. These are all going to be variants of the same light. Single XML LED, multiple brightness settings, 18650 batteries.
Cave Adventurers - Explorer back-up lights - Marianna, Florida USA - Never Undersold!
I have a pair of these, and they are great. Button on the head has a LED indicator, 3 levels of light, uses 18650's etc. They're a bit expensive, but they are really nice. I would recommend getting both in spot versions for poking around. You need multiple light levels with lights this bright for night diving so you don't harm the wildlife.
These lights come with a battery and an individual charger. They work fine, but I'm not a fan of either of them personally. Charger doesn't give any indications on what it's doing other than charging or not, and the batteries I'm not entirely sure I trust... I use them, but they don't last anywhere near as long as my other ones and I suspect they'll need replacing here shortly.
https://www.amazon.com/SecurityIng-...1492784176&sr=8-2&keywords=securitying+diving
I have one of these which is a great little light. Button is at the back of the light, but no battery level indication. Does not come with a battery or charger. Stuart lost one of these to a sharp edge on the end of the light when it sawed thru the cave line. I haven't had this issue, but I did go back and fill all of my lights holes since they were all a bit sharp.
https://www.amazon.com/KeepPower-34...TF8&qid=1492784425&sr=1-16&keywords=ncr18650b
4x high end batteries here
https://www.amazon.com/NiteCore-D4-...d=1492784495&sr=8-6&keywords=nitecore+charger
nice battery charger there.
Total for 2x lights, 4x batteries, 1x charger is ~$140.
1-R105T Handheld Light – ANO Online Shop
I also have 2 of these lights. They come with nice batteries, but meh chargers. Button is on the back, no battery indication, but they're really nice. $160 for those.
With either the Explorer or ANO lights, I would recommend at least getting the nicer charger. The explorers would get a bit of a nod from me due to the battery indication as well as the better domestic support. Those guys will definitely take care of you
bolt snap how-to
rectotec: HOW TO: TIE A BOLTSNAP