If staying 4S is no problem for you, you could say that it's the perfect way to go waterproof with the ERBE, isn't it?
It's proven to be truly underwater-waterproof, rated for 120A continious, works with up to 4S LiPo and a max of 3000 KV brushless motors (with 4S).

So wouldn't it be a good idea to hook it up to the Mamba Monster motor and 18/54 gearing and have a waterproof brushless E-Revo?
I can't find much about that combination, so either it's not such a great idea, or no one has really tried it yet...?