There are different problems. The windings could consist of Copper wire with different coatings. The Iron core of a outrunner consists of thin iron plates which corrode fast but could be impregnated or at least passivated before caulking them together. It depends.
Why is it good to use some stable coating for the windings? When the water is between the winding packages it generates higher forces at high RPM than air. So if single wires can move, the isolation might break down. You get a connection to sea water or direct short between single wires, the performance will go down. So one issue is to improve the stability of winding package.
Iron core: The thin plated iron is corrosive, passivation does not help too much. It is done to reduce eddy currents and to build up a kind of insulation between the layers, it does not need to be perfect and caulking will open the passivation layer at every lamination. Sometimes the core is also baked afterwards to remove mechanical stresses. Anyhow, it is surely not seawaterresistant.
The problem i see with such sprays, that not every cavity can be reached by such process.
So somehow the air has to go and something else shall take place.
How can you achieve this? Frequent vacuum. It is not enough to apply it once, you need to apply it several times to expand the air in the cavities by lets say factor 2, some of the air will leave. Now you apply normal pressure again, the air will collaps, the fluid material will go in a bit. Next time you apply vacuum you have less air coming out. Repeat until you do not see any air coming out.
I am sure, such silicone release spray is a good barrier, but how can it reach these cavities? By capillary force only? I think it would dry on its way. Check it out.
I will write some more details in my thread about the details building vacuum chamber cheapiest way.