Thank you all for the comments and product suggestions. Silicon crossed my mind as its the easiest but I'm not a fan for reason listed above and the abuse these things take, add in salt water dives and over time things will degrade. I like the products suggested and the oil method too.
Yes there will be testing done, obviously. This thing would have to survive a battery (pun) of tests before I would go near it physically. Most cave divers don't dive to 300ft, that's about the limit for even some serious hardcore divers, the average depth at least of the caves in Florida (yes believe it or not Florida is a destination place in the world for cave diving) ranges from anywhere from 30ft to 160 feet. Those who dive know once you start breaking the 130ft mark time is ticking and you are gobbling oxygen, unless you are planned and certified for a staged decompression dive which honestly I haven't seen almost anyone do in Florida caves except for certification purposes.
I had thought about using some of the MG black colored sealant (1 part mix). I forget the part #. I have to do the math on the amp draw, I'm honestly feeling pretty ill right now so I haven't done it. But the specs on the website are as follows:
lights come standard with a 5.2-amp hour li-ion battery pack
~80k lux at 1m (3ft) -1550 lux at 5m (15ft) on high power (using the standard, 5.2-amp hour battery)
Approx. 2.5 hours burn time on high and 5 hours on low (using the standard, 5.2-amp hour battery)
when I look under there battery packs for the lamps they list:
3S1P Lithium Ion battery pack
5.6-AMP
Charging voltage 4.0 A
11.1 Volts
62 Watt hours
Then they have their upgraded pack which is 20ah which they give very little info on its just a longer tube housing essentially:
Approximately 240 Wh hours
800-1000 charging cycles
I know I could blow these specs out of the water (pun again!) with better batteries. They are just stored in a simple hard plastic canister and the battery appears to have to contacts that mate with the lid and then there's a pack of some sort. Nothing is oil filled or anything that I can tell from limited pictures, may I will try youtube to see if anyone has torn one down. I remember when I owned one of these lights (much older model and made by the original company that created them until this company did a hostile takeover of the other company. We would have to remove the packs and plug them in to charge. I wish I still had one. But I do remember flooding the thing in a dive, of course we have triple redundancy in lights per diver and buddy so I wasn't in danger of being in the dark, and I was trained for that situation too. But a dive light is life support no doubt.
When I surfaced it was just full to the brim with water. Nothing happened. No boom, no fizz, nothing. Of course back then I wasn't into electronics so its not like I tore it apart to see how it was built. I just bought a replacement battery and was good to go. Several other divers had the same issue.
I had though of maybe using headway batteries, but the compression concerns me. Curious on battery chemistry types. Clearly the company has gone Li-ion.
Another dumb question I have seen videos of people taking a fully 4.2v charged Li-ion battery (unprotected flat top) and just drop it in to water. Nothing happens. Why? Or do the videos just not give it enough time to finally corrode and then the fireworks happen? The only time I've seen them instantly explode/catch fire is if the person physically damages the battery ripping it open and removing the sleeve. That's not much of a surprise everyone know what lithium does when exposed to water!
Anyways, you guys have given me some ideas and thinking to do. I'll be trying out various chemicals and oils as mentioned here to see. Once the pack is built the first test will be the worst test, drop it down to 300feet and let it sit until the battery runs out. Then reel it up and inspect. If its still in one piece and I am able to charge it safely, then more tests and more tests and more tests
I'm also stuck on BMSs I normally use high end BMSs but they wont work for this cylinder shape I can only think of the cheap Chinese bms's which scare me. When I'm feeling better I'm going to go pick up some black heavy duty PVC pipe to at least carry out some initial tests. I've got some sink holes around here that are about 60 feet. which is a good start for just checking my ability to provide a watertight compartment. I guess I will likely start with some 21700's. Man the BMS is killing me. I don't want cheap junk in there, but I can't fit a square BMS in a cylinder! Changing the pack design to any other shape we turn off anyone. Cylinder shape have always been used because of the ability to mount it almost anywhere with no interference to the diver. I can't strap a big old square or rectangle to them.
Sounds like this is going to be fun
to home depot to buy some parts for the cylinder, and once I'm feeling better I'll do the math on the batteries ect.. and pick out some that will mop the floor with what they are providing. And if I let the magic smoke out at least it'll be underwater tied to a cable so no injury to anyone! Why am I adding more projects to the mountain I have!?
Cheers
any further comments welcomed!
mgchemicals.com
. Just one tiny hair and you had a flood. I would think that someone already makes a depth rated high output by now or they are insanely expensive. Just keep in mind that if you goop/jell or seal it up, you will not be able to take it as luggage on a flight 
dang that would look suspicious. I miss the days of pre 9-11. 
