With ardusub there is QGC function named “input_hold_set” that you can allocate to a joystick button. When your drone fly a certain way, pressing this button fix the condition and the drone keep moving at constant pace. For example, if you move at 0.5 m/s and press the hold set button, you can then release the throttle and the drone will keep moving at 0.5 m/s.
This is handy for many survey situation on water.
Is there a similar function in ardurover / arduboat? Or do you know an alternate way to achieve this in manual mode?