I h ave used several coatings -- to water proof --- pool paint , sort of rubber like --

One thing is the concrete has to cure good before painting otherwise the paint doesnt seal.

THe best way I found is to get about a gallon of fiberglass resin and hardner and use this to paint with. It seals everything real good.

If you are casting in the ground always put a poly liner first than lay concrete. This really retards leaking and is the first line of deffense.

Best regards

Ed