Maybe since you already have a depth (pressure sensor) you don't really need a differential unit, just a single pressure sensor on the front to sense speed. It's speed calculation could be based on the depth reading. This pitot tube could probably be mounted on the above the hull as well. What do they call the thingy that has the periscope in it?

For stealthy sensing, you could try to find a smaller one of these http://www.blueheronmarine.com/Airma...k-Plastic-7241

or design one yourself, but this takes it up more than a few levels on the difficulty scale.