I’ve tossed this idea around too… heat and torque are my concerns. The mast would experience high torque, especially when starting from a stop. An engineer explained it to me that the force is as if the prop is being held still and the motor is trying to turn it, the mast would have to hold that torque.
The 90 degree gearboxes have lower efficiency which will generate a lot of heat. In theory, that can be dissipated under water.