If you run in an open space, then get a fast motor like cripplethreat is using. I run 17.5 so can't help on gearing but he just gave you his gearing. Get a couple pinions in that range like 24, 25 & 26 so you don't have to order again if a 25 is not right for you. All motors are a little different and where you drive it will determine correct gearing. Gearing for an open lot will be different than for a track.
I don't look for waterproof anything for a RC car... water is the enemy. Bearings hate it, screws rust, electronics like you found out really hate water. Also just because the ESC says waterproof, that doesn't mean the servo or motor is waterproof. If what you happen to get is waterproof, fine but don't make your buying decision on it being waterproof.