Turns out it’s not too much of a problem. Graeme from RGU said if you make a point of drying out the motors and lubricating them after each use, they have pretty good lifetimes. Antony, another UK ROV builder, tried this same trick with 550 frame motors and found it worked remarkably well. So I followed their advice, and didn’t find much in the way of appreciable wear.
I wound up loaning out my ROV about a year ago, which is why there are no pictures of the ROV with its buoyancy installed. The “dry 'n lube” instructions never made it out with the ROV, so I’m almost certain they’re just dunking it, running it, and putting it away wet. But I heard back from one of them a few weeks ago, and it’s apparently still ticking right along.
So here’s where I get REALLY REALLY fuzzy: Anyone ever water dipped a motor to wear the brushes in? Run it at low voltage, watch the water turn gray, pull it out and see if you get a nice seat of the brushes to the armature, that kind if thing? Electric RC car guys do it all the time, and with the 550 frame motors. So I know there’s supposed to be very VERY fast wear. (Water dipping typically takes five minutes or less, usually less.) I figured after a few hours of operation my little dinky motors should’ve been fried.
Apparently they’re not. And the first time I really used it, I took it to Robofest in Hilo, tossed it into one of the ROV demonstration tanks, and handed the controls to the nearest kid. It made the rounds 'till the batteries died several hours later. I was mighty surprised to find the brushes had hardly worn at all.
So the short answer to your question is no, the motors don’t get thrown away after a few uses. The longer answer is all of the above plus the statement that I’m 100% certain I am missing something. Because everything I’ve seen indicates they should be dead.
Got a fun follow-up on the bare motor in the water idea that’s a little easier to swallow: Antony, that same guy in the UK who’s been running larger motors bare in the water has also used brushless DC motors. He epoxy dips the windings and all solder points, but then just reassembles them. No brushes, no sparks, no real wear at that point. A brushless DC ESC in the dry box in his ROV then drives the motors. He’s got extra stuff in there to reverse them (most brushless DC ESCs don’t allow for reversing), and they work like a champ.