Don't believe it's condensation. condesation won't rust to the point it sounds like you're talking. It is repairable unless the rust has caused the the magnets to turn loose. Brush springs will be rotted. If armature shaft is pitted, then it will eat the seals. Will need to be resealed, and since you can't find a place for seals to fail, the shaft should be removed from the motor, threads cleaned, and re locktited.

Also, a DC electric motor may run underwater without seals, but not for long and not right. The water will will cause a problem in brush contact with the com bars, making the brushdust an insulation unable to carry current.
John