That could be similar to my experience. I replaced an old Nexus transponder with a DST800. Often now when I put it in it reads one or two metres depth when I know for sure that it's around 20 metres deep at that spot. After some trial and error I found that in my marina pen there are long strands of seaweed (growing on the marina finger, and not very deep) which reflect the transponder's signal when the tide pushes them under the hull. Out in clear water the transponder works fine. On a mooring it should be OK I think.