Night watch

Some things cannot be explained
Our watch schedule has Kurt and me on deck from 8 PM to 2 AM and Mark and Andy on watch from 2 AM until 8 AM. Kurt and I split our watch so I take the first 3 hours and he takes the second 3. Tonight, I am still on deck and it is almost midnight. The stars are like a blanket over the boat. We can steer easily by keeping Orion just to the right of the mast. The North star is over our starboard beam (right side) and the Southern Cross is over our port beam (left side). The Milky Way is a wide ribbon of light overhead that stretches from horizon to horizon.
Now here is the strange thing. The charts say that the water is between 3500 and 4000 meters deep. That is approximately 11,000 to 13,000 feet. Most of the time the depth gauge just has three dashed lines indicating that the depth is greater than it can read. However, every now and then, it will register a number like 3.5 feet or 5.1 feet. Since the keel of the boat is deeper than that so obviously it is not something that stretches the width of the boat. Of course there could be a large fish or perhaps several smaller fishes swimming along under us but there is no way to see if there really is anything under the boat without going into the pitch black water – it’s not likely I will do that.
Does anyone know what would cause such a reading to occur?