If you dig into this I think you should take this into account: my data analysis so far lead me to think that the depth sensor is not precise enough (specially just below the surface) to be of any use in data filtering. Also, if depth is still the solution, then my ROV depth sensor was about 20cm below the surface for the antenna to have a fix. This depth will change from user to user and with water condition (waves, wind…). If Depth is to be implemented, it will need some sort of live tweaking in QGC. Unfortunately not a set and forget thing. This is why I think Valid Sat Count may be a better signal indicator and filter.
thanks