I just finished a competition for underwater robots. The water is about 40-50cm in depth and 90% of the designs just exposed the thrust motors completely in the water similar to the picture below and nothing happened during the competition. BTW, the water is tap water, not salt water.
I saw many questions and how-to articles about sealing the motor shaft for underwater ROV. I am not sure I understand the reason, especially after I found this reply to Preventing leaks in motor shafts for underwater bots. Is it because of the concern of short circuit or wear and tear if water goes into the bearings? Or will things be very different when the robot has to go deeper?