Most closed can motors have very hard brushes, that will soften a bit if broken in under water at a low voltage.

Don't overdo it because you'll wear out the brushes faster than normal.

Also let the motor (and the brushes!) dry out thouroughly, before oiling the bearing lightly and using it.

The better contact between brushes and commutor will reduce sparking (interference) and give a longer motor life.

Regards, Jan.