In our sub, we tested the drivetrain with the RD on land and didn't have problems, but if i recall correctly we didn't test it with resistance on the propeller, so that may have been a problem. Also its possible the RD wasn't in an ideal position and there was less chain wrap on the cassette. I think in our case it may have just been a perfect storm: viscous environment, old chain+cassette, lots of resistance, poor geometry, etc.