I’ve used other brushless motors (camera gimbal motors, in my case) as the electrical generator in prototype marine energy harvesting systems. They work great if you can get them to spin fast enough. But even a well-designed turbine in a water flow doesn’t turn very fast compared to how fast the T200’s motor spins.
The blade tips on a well-designed rotor move about 7 times faster than the speed of the current (as long as there’s plenty of torque available to turn the rotor!). (Fig 5 here: https://www.nrel.gov/docs/fy12osti/) With a 1m/s (1.9 knot) current and a 50cm diameter prop spinning with no load, this would translate to an upper-limit propeller rotation rate of 260RPM. If you attached that directly to the M200 motor, it would generate about half a volt, and that would only allow you to extract a tiny fraction of the power the rotor would be able to supply with a properly designed high-torque, low-speed generator.
Properly loaded, a 50cm diameter rotor in a 2 knot current can theoretically deliver about 60W with no drive system or electrical losses. Probably going to be more like 20-30W with the mechanical and electrical losses in a small experimental system. But to get there, you really need a speed increaser. In my application with the camera gimbals, they were all in a sealed container already and I used a 50:1 gear drive.
It’d be interesting to see how big you could make the tiny trickle of power you’d see out of a direct-drive M200 motor with a practical prop but I think you’d need an active rectifier to convert to DC.