I've caught otter in a marina using a white 5 gallon pail and a 220. Drill some small holes in the bottom of the pail. Put some notches in the top of the pail to hold your 220. I run an 1/8" cable through the side of the bucket and then connect it to the trap, this will allow you to pull the trap, bucket, and hopefully the otter in. Place a good sized rock in the bottom of the pail. Then for bait I use a large silvery fish. Wire it to the rock and to the bottom of the pail. Guard the pail with your 220. I set the pail in the water and let it flood through the holes in the bottom. When the pail starts to balance I push it out into deeper water. Hopefully it sinks to the bottom sitting upright, and without setting of the trap. Hopefully this will give you a chance to get your problem otter.
That's an awesome idea! I have a an otters-at-the-marina problem too! My big worry was trap theft with cages, as anyone can walk out onto this un-guarded, un-gated dock. I bet a bunch of people will walk by, never even thinking my traps are underwater tied to a belaying cleat!