Personally I dont agree with "water proofing" as in sealing the esc. If it's not perfect you could end up trapping water and ruining the esc over time. I would just use a balloon. It's hard to really seal something. To me it's better to mostly seal something and have the ability to air it out.
Recently I started using balloons on my esc, rec pack, servo and receiver. No problems at all. I use zip ties to close the balloons and stick velcro right on it, so I can keep it them in place. But the big thing is take the electronics out of the balloons to make sure theres no water, I do this every 2-3 runs. You have to base this on how wet your boat gets. Also I just bought regular party balloons, some cut, like the servo balloon.

pic attached(yeah the esc isnt in a balloon at this time)