DC motor is more "electric", than "electronic".
There is nothing in a motor that water can damage.
Do NOT get water near your electronic ESC though, it will fry.
Same with RX, TX, servos etc. Remove all that and, heck I
bathe my cars in the bathtub.
Running the 540 in air for hours will break in the brushes, but it
will also wear out the bushings early. Running it in water will
break in the brushes faster, but the bushings won't wear out
so fast. The water also causes resistance for the rotor, and is
a good conduit for heat generated. The water will go grey with
all the brush filings.
As said above, completely DRY the motor and lube the bushings
before reinstalling in your RC car. Wetness will promote rust.