I read that some cavers filled the inside with PJ or some other similar "grease". Deep submersible vehicles will use pressure compensated battery compartments that are filled with mineral oil and have a diaphragm to the ambient pressure.

Bill

A Photon II doesn't have any circuit inside; it's just an LED connected across a couple of coin cell batteries, similar to an incandescent flashlight. The LED leads make contact with the batteries when you squeeze the light. If you dunk one in water it's not destroyed but if the water is impure (conductive) it means that the light can (dimly) light up and this can drain the batteries. I've seen that happen after people swim with the lights clipped to their clothes. The light is fine after you rinse it out with fresh water, dry it, and replace the batteries, but that's not so easy in an emergency. That said, a Photon II will run for days on end while still producing enough light to keep you from bumping into things in pitch darkness.

The Photon Freedom does have a circuit board that's subject to destruction by salt water, but it's better sealed than the Photon II.

I've heard of someone recommending sealing the seam and switch hole in a Photon II with vaseline, but I haven't tried that.

The heat-sealed baggie thing sounds like a good idea, and you can also get small ziploc bags that should be pretty waterproof.