Waypoint Navigation Error with surface GPS

Hello @EliotBR, @jacob and everyone,
we are facing issue with waypoint navigation in qgroundcontrol, our ROV surge down even if we give the altitude 0.0 m while defining waypoint in qground control’s plan view. we also tried different altitudes like 0.5 m,1.0 m, 1.5 m, -1.0 m and -0.4 m , in all cases the ROV surge down very fast rather than following the waypoints

we are using a surface gps, a waterlinked a50 dvl ,navigator(ArduSub V4.1.1) with qground control v4.2.8 and blueos 1.1.1

please suggest whether this method is right( in below image) to give depth value with force save enabled.

Also i connected another gps(bu-353N) with my ground computer so that, the surface gps connected to ROV will have a reference point.

But either the case, the ROV surge down very fast as soon as i start the mission.