I don't think brushed motors would be any better off in water than BL, in fact probably the opposite. The brushes wear down much quicker when wet which is why people dunk them in water to break new motors in. Add to that, water is conductive and there's a lot of exposed current carrying metal in a brushed motor.

Agree with lutach. Most BL motors will probably be ok in water if you seal the motor connectors. Salt and/or dirt could be an issue though.