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.
> 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.
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.