you see if you apply liquid resin properly, water wont enter through shaft also. it all depends on how well you apply the resin. although you may go in for coating the internal gears and circuitry inside the servo for more protection but I dont think that would be required if your external coating is effective.
and battery you can easily find for driving your servos based on their current ratings. I’d suggest you go in for two batteries rather than a single battery to drive all the servos.