Thanks for splitting this off the ESC thread.
Your assumption of 300 Ohm body resistance was really surprising to me. I read the article you linked (for others, the relevant experiment starts from page 11) and checked their sources. Especially „Electric Shock Hazard of Underwater Swimming Pool Lighting Fixtures“ (DOI: 10.1109/TPAS.1964.4766095). I did not expect the drastic loss in skin resistance when wet. ( For the lazy: In the first paper humans were connected through fresh water, in the second one through really salty water. First was 12.6mA@4V with comparable results in the second one.) The sources appear to be quite old. I didn‘t find newer ones till now but the physics should not have chnaged since then and the research comes from reputable sources. Would be nice to have some data from experiments with dc insted of 60Hz AC. Thanks for bringing this up, it‘s really interesting and it seems to be correct.
Assuming your electronics are watertight, and your submerged motor develops an isolation fault. Even if multiple windings lose thier isolation the current should still be flowing in the copper as this is a path with lower resistance.
If some of your windings fail open circut with the loose ends connected to the water it should still be safe, as you have mentioned, bescause there is no second contact. I do understand your concerns with unisolated kill switches in the ESC thread now. Those could be a second contact.
Concerning kill switches:
I would not recommend switching real power directly. The kill swith should only pull a signal via tens of kiloohms to ground, which in turn switches your main battery disconnect.
Detecting isolation leaks:
In completely isolated power grids usally a decice for supervising the isolation is used. Without such a device an unharmful first failure can not be detected.
It seems to me such a device would be useful in efoils, too.
I‘m still not overly concerned, so I probably would just install a warning light. You could of course disconnect your battery.
Supervising the the isolation could be as easy as connecting a water electrode to logic voltage via a high resistance and monitoring the current with comparators. On a predetermined threshold the battery is disconnected.
Monitoring the isolation could be really useful, detecting water ingres before something breaks sounds nice to me.
Keep your suggestions coming.