Aiming with the speedboat's GMG is very difficult, especially at rough sea. It seems more realistic that such systems would be fitted with a vertical stabiliser like a tank, in which the gun moves independantly from the optics but tries to maintain the aimpoint as much as possible.

This would *NOT* make the weapon ultra-accurate, since it still would have a slight offset between the movement and the correction, but it would make aiming less of a spray'n'pray activity.