Okay...I can shed some light on this.
You use mineral oil. I fill my light head and it helps disperse the heat (Flash Point > 420F)....in fact, I can run my light out of water as long as I need and then jump right.
I also fill my canister up too. My canister is small and uses NimH batteries so it only adds 1/2 pound to the weight since there wasn't much air to begin with.
My light is all homemade so I was concerned with leaking, especially with $40 worth of NimH batteries that would be corroded to hell if they got wet. So I sealed it the best I could and then filled it with Mineral Oil. I drain the oil at the end of the year and I usually find about a teaspoon or less of salt water.
Charging is easy. I have charging wires coiled up in the bottom so I just pop my end cap off and grab the wires and charge it. NimH batteries don't vent so there is no problem for the batteries....just make sure that you don't quick charge or overcharge because you risk popping the vent on the battery and ruining it. The oil doesn't ruin the batteries in this case because they would dry out after that vent pops anyway.
Now, all this said, I would not recommend using sealed lead acid for this. Sealed lead acid batteries have rubber caps on their vent holes that allow pressure to escape but not go in. Under pressure they do have air in them and the oil will be forced inside. This doesn't ruin the battery but does shorten the life I've found. I got about a year out of my lead acids using the mineral oil method. Considering how cheap they are, that's not all that bad.
Another tip, my lead acid canister is 6" round so when you fill it with oil it does add 2 to 3 pounds of weight. I always strap a 2" PVC tube sealed with air in it on the side of the canister to balance out the weight.
Mineral oil is a little more messy, but it reduces the failure rate to nothing.
Note** Mineral oil will basically allow you to use any electronic device underwater. I do quite a few night dives and got tired of the weenie strobe lights you can't see in low vis, so I place a high power strobe light (75 Watt) into a mason jar of Mineral oil and hang it on the line.
~JT
http://ecra.emeraldcoastboating.net