One good thing about brushed motors- As long as the voltage isn't too high they are already "waterproof".
Just need to make sure and clean/dry them out after every run and they'll be fine. Mudproof is a different story, as the sand getting in the wrong place can (and will over time) ruin the commutator/brushes in the motor.
Still, "standard" 550 motors are pretty cheap, so if they last a season then
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Edit: Run a pair of these on 14 cells and I think you'll be pleased.
http://holmeshobbies.com/product.php...&cat=17&page=1