Helps to know the application.
How about something like one of these:
You can find these on eBay or at Electricscooterparts.com for around $100. This one is called a MY1016Z2, made by Unite Motor.
This is a brushed motor, which means you can easily reverse it and you can run it straight off the battery without a controller.
Ad indicated output is 400 RPM at 24v. For your use, it might be fine running off a 12v battery and it would go about 1/2 speed.
These are not waterproof, but if you sealed the housing with silicone and packed the gear reduction with grease, it would probably survive a short time underwater. The idea of mounting it high and extending the chain is a good one. You could even just place a plastic container over it so when it goes into the water, an air pocket is trapped in the container where the motor is, like a diving bell.
"One test is worth a thousand opinions"