One of the main reasons we've been able to keep the cost of the ROV much lower than any of the commercial counterparts has been the use of exposed brushless motors. We've experimented with a number of different configurations and modifications, and are always learning better ways to maintain and adapt the motors to work in water (especially sea water). A number of community members are experimenting with ways to improve this design: Mark's coatings, Ben's magnetically coupled drives, etc.
On the forums, Armand and Kjetil noticed an issue that his motors shorted when placed in the water, and that it was coming when the motor mount came into contact with the water. The problem was new to us, and we tested it on a batch of 200 motors we had around the lab. We found that a small percentage of the motors had a short between base of the motor mount through the iron core and into the stator winding. We're still playing with the motors to see if we can further isolate the issue. Follow along on Armand's original post.
Meanwhile, please email me (firstname.lastname@example.org) if you are experiencing similar issues with one of your motors.
If you're still in the building phase, you can test your motors by putting a multimeter to the base of the motor and the end of any of the three leads (see images of good motor tests below). A bad motor will range anywhere from 0.1 - 200 ohms. Again, feel free to email me with any questions or if any of your motors are bad.
If you've already assembled your ROV, you can check the motors through the DB-25 connector.