Your mag coupling looks like it should do the job.
I’ve only managed to test auto missions on the bench, but when I change the mode to AUTO, the thrusters take off and it wants to try to navigate to the waypoint. Jacob has mentioned that there might be an issue with surface vehicles and waypoints: Way point following issues for surface vehicle - #8 by jwalser I’m at a hard point because I want the manual control and sensors from ArduSub, but the dedicated 2D waypoint and control options (skid steer and conventional) from ArduRover.
Because the Simple-3 frame uses servo mixing in a default skid steer, you would need something like a reverse servo mixer to take the servo outputs and convert them back into a throttle and rudder control for your USV.