I was using a light in the rain and wondered what happens if an o-ring is missing or whatever and water does get in. So I let some in to see. I took my most expendable light, held it under water and pumped the zoom till it was full. Some leaked out, in two days, as can be seen in the picture.
This has a boost driver, so there is only 1.5 V unless it is on. A lithium ion battery will hydrolyze water, making metal hydroxides, but 1.5 V is not enough enough to separate water.
Today I had to work the switch a couple of times before it came on, usually a sign of corrosion.
You could try filling it with alcohol 
I tried that before, for cooling. The main problem was that it softened the epoxy I had put it together with.
Don’t you just hate it when ypur epoxy goes soft…
We divers know saltwater has a rather more spectacular effect
Yes, the next step may be to add sea salt.
When I abalone dove, I carried but never used a very cheap Pakistani folding knife. Disposable rather than immune. If, for example, a Sipik SK-68 dissolves an hour after you come up, that is no real problem.
If your going to trust an SK68 underwater your in big trouble
I worked for only a few minutes.
After it stopped, it worked again when the water was emptied out. It was dark and drew only 0.05 A at the tail while full of salt water, and and it lit up and drew 1.5 A when emptied. My guesses are that the conduction between parts of the three mode boost driver either deprived the control circuit of power or stopped the oscillation. Perhaps a one mode driver would work. A one mode direct drive light would surely work full of salt water, but it seems there would be drain and corrosion on the 3+ V battery even when off.
I tried a equally expendable one mode that Popkind sent instead of something I ordered. It behaved similarly, so it looks like boost drivers don’t work with small shorts.
If one really wanted a cheap light to work in the ocean, the next step would be to pot the driver.