300Ohm is a good value to calculate with for a body in salt water. 10mA is the threshold for harms. So if you swim in the water being e.g. energized by a slowly moving BLDC motor having a leakage will result in harmful AC conditions if you touch an e.g. grounded start or kill switch connected to battery. So the harmful voltage is as low as 3 Volts. 30 V is deadly. Anything between will give high risk you cannot control the situation, e.g. because you cannot breath anymore.
We should face this risk by fault tolerant design.
E.g. there should be a solid barrier between the interior containing battery, ESC and so on and the outside, preventing the closing of the loop. The only outlet should be the 3 power lines going to the motor. If there is a single failure regarding any cables, connections or the motor itself the loop is still not closed, so no harm is possible.
Water cooling the ESC is an issue, if the cooling circle is open, so water is exchanged. There have to be measures against any water ingress into the housing of batteries and electric circuits. The name “Kill switch” gets another meaning. Most kill switches have a very simple design with a sealing membrane between in and outside, being clamped by a force built up by threads. It can become untight if the thread is not tightened well, which can happen easily if you do not support the switch at the correct nut when tightening the outer ring to seal the bore. So reed contacts might be the right solution to substitute a kill switch.
To overcome the risk by a broken battery box, the motor and all cables and connections have to be sealed very well. Also encapsulated inrunner motors pose some risk: If there is water ingress to motor housing, any electric conducting part like connectors or flexible cables or solderings being wettened lead to a conduction to the ambient water by the outer housing. Also burning the motor windings or any cable inside the mast has high risk to make contact to the water.
Please contribute in finding solutions.