Honestly, what it may be...
In SH3, harbor traffic can do odd things... Docked ships drift even though they are "at anchor" and sometimes sink.
Can you describe the problem more? Are all ships that you see sinking... or just a few of them?
It may not be a sensor problem at all... but just another SH3 problem.
Just a note to all... If you intend to use the late war snorkel sensors in GWX... READ THE MANUAL CAREFULLY!!!