One more thing: That the motor is not suffering from water does not mean, that you are protected against electric shock. Risk is a function of voltage and salinity of the water. If you have the risk that there is a connection of a motor phase to the water, you must have additional measures to protect against electrical shock, e.g. a completely isolated housing for the batteries and ESC and cables with no electrical connections to the ambient. To run electric current through your body you need two poles. If these two poles are inside the motor it will only harm the motor, not you. But if you touch a part on your battery enclosure with a linkage to battery + or - or a 5V or what else derived from the battery, while standing in the water and having a leakage from the motor phases, this could kill you.
This is also a risk in case of accidential damages of your
rotating or stillstanding thightenings,
battery or ESC case,
sensor cables and sensors, even hall sensors,
The problems arising from these requirements are hard to fulfill at any time under all circumstances.
With voltages below an unknown value (i have searched a lot for internet information, but did not find any recommendations) the risk is neglectible, you might feel something strange in your finger. At this event i stopped testing and got things done better. But the risk is evident, that the current could rise and harm you deadly. Especially if some mechanical breakdown occurs.
So the standard rule is: Protect anything metal (and carbon) and all cavities and build an isolation around all electrical parts. By this at least you will keep the leakage mechanically small and thereby the electric resistance high. A small crack or hole in some coating has a different resistance than a tinned wire hanging loose in salt water. If salt water enters the motor compartment and get contact to e.g. flexible cables, there is also a contact to the inner aluminium wall, of the mast or the fin box, any plug and cable needs to be tightened.
An additional measure could be a detection system which can measure the leakage, but for this, you have to install some anode into the sea water, connected with very high resistance to the battery. But i guess, that a simple reading of the voltage over the high resistance could deliver a lot of positive false results. So there is needed some intelligence, some considerations and before all that testing.