I HAVE THE SOLUTION!!!
Get a Mosfet! the fet itself can be made watertigh with a bit of sealing silicone (like in your bathroom, around your bath) The motor can run completely submerged (yes I tried it, in fact I run my new motor in water + alcool for 3 minutes.... help shaping new brushes)
Battery can be made watertight too with the help of the same silicone. everything else, just clean-up a bit after... that is all