Since there seems to be some interest on this subject I’ll describe an experiment I once did.
I wanted to learn more about the failure modes of a light converted to a dive light. I used a cheap Rominsen RC-K4 (or something like that). It was a 3 AAA XR-E light with a reverse clickie.
The switch was removed which left a hole in the tail which I filled in with marine grade epoxy. This left the light as a “twisty” by screwing in and out the tail cap for on and off.
The front lens was very thin, maybe 1 or 1.5mm. The o-ring in the head was useless both because it was so thin and because of its location. It was around the lens in the head rather than behind the lens so it really did nothing to keep water out except for maybe light rain. It was more like a gasket than a compressible o-ring.
I removed the o-ring and used marine grade silicone sealant around the edges on both sides of the lens.
Then I tested this light on a night dive to 100 fsw (feet sea water). I expected it to fail but I wanted to note at what depth it failed and what exactly the failure modes were. I had another light with me of course (2 more actually).
Initially, I turned the light on before I entered the water to eliminate the act of twisting the light on as a failure point. I check the front lens every 30 feet or so on the descent to 100 fsw to look for leaks.
It never did leak! At 100 fsw I decided that I had learned all I needed to know and was therefore willing to twist the light off and then back on. It still didn’t leak. Twisting the light on and off at 100 fsw was the worse case scenario (for this test).
The lens was thin but the head diameter was only 30 mm or so and thickness is a function of surface area for these purposes so it worked but wasn’t ideal.
I didn’t bang the light around or bump the lens against anything underwater or it may well have failed.
That’s what’s not generally tested in a pressure pot…dynamic pressure rather than just static pressure.
Rarely is the issue (for leaking) with a light the body tube head or tail connection. Almost all lights have one or more o-rings there and long enough threads that there is an effective seal. Grease is helpful but that isn’t what seals at 100 fsw or at any real depth underwater.
Switches that are desirable in non-dive lights aren’t desirable in dive lights due to increasing water pressure which would activate the switch. With a reverse clicky water pressure would keep the switch from ever turning on.
Due to the way these switches are constructed water would almost always leak (flood) in at some depth and that would occur fairly quickly (at a fairly shallow depth). Spring pressure could offset that initially.
A thick front lens is desirable but it may not initially be the cause of a leak (may not break initially if not banged around).
Many dive lights are spec’d to 300 fsw so they can be used in reality down to 200 fsw or less.
I’m sure (pretty sure) that the Rominsen would have failed (lens broken) by 200 fsw. My regular main dive light has a lens that is only 3.5 mm and that’s with a diameter of close to 50mm. I wouldn’t be comfortable with it at 200 fsw.
People have had them down to 150 fsw or so. I’m had mine down to 130 fsw. The light I had before my current main dive light was taken down to 200 fsw several times and I had no problem doing that but it was thicker and all plastic (HID not LED however).
Your average flashlight may do OK in a swimming pool (or it may flood) 
That’s an addition 1/2 atmosphere at most. Beyond that most lights would fail (and/or not work) just because of water pressure and the clicky switch.
I don’t know exactly how a Zebralight (for example) is constructed but conceivably it could work with the electronic switch (not sure about how the switch is sealed) however it would be continually changing modes due to water pressure but otherwise it might make it to 100 fsw. I’m not going to test this however!
(I think primarily on this one, and there is Yezl Q2 for example on cnqualitygoods…
