There are some more tips on preparing an outrunner for use in water in this thread…
One suggestion was to use liquid electrical tape on the wiring solder joins at the motor, or plenty of silicone inside heatshrink.
Coat the stator thinly with epoxy so that it can be cooled by water. Fill the gaps between magnets on the bell with epoxy (with microballons) to protect them from rusting and reduce drag.