[hunter2_]

Seems like it should be possible for the phone (not the battery compartment and battery door, just the rest of the phone) to be waterproof, and for the battery to be waterproof, and the two can contact each other regardless of that contact being in a wet location. After all, we have plenty of electrical wiring methods rated for wet locations, so why not this? Water isn't the very best insulator (especially saltwater and other mineral content, which the contacts would need corrosion resistance for as well) but it should be sufficient at 4VDC.

[eterevsky]

Wikipedia says that the resistance of water is 0.2 Ω·m for sea water, 2 to 200 Ω·m for drinking water. This is very low and can drain your battery almost immediately.

[bombela]

Wikipedia also says that water starts conducting at 1.23V, and the common Li-Ion nominal voltage is 3.7V.

3.7V - 1.23V == 2.47V. 2.47V / 0.2Ω ~= 30W (12.5A).

You will be able the find your phone on the seabed from the bubble emanating from it.

Though the battery protect circuitry (which is inside the battery package) will most likely cut power before the battery is risks damages.

[hunter2_]

> protect circuitry (which is inside the battery package) will most likely cut power

I wonder if that's responsible for the experiences folks had way before waterproof phones were introduced: they would drop their phone into water, it would shut off and refuse to boot for a significant amount of time (enough to send them shopping for a replacement) and then they'd try it a week later only to find that it works fine. Accelerated by putting it in a bag with rice or other desiccant.