Sounds like you should replace your impeller as a precaution, as it is the most likely culprit due to its wear & tear. If your diesel is fresh water cooled I would verify good flow from the exhaust of the heat exchanger. It can be hard to verify water flow out of the exhaust port on the transom while underway, if you have wave action astern. The transom usually is covered with sea water. Where is your boat located on the York? My C-30 is at Crown Point Marina on the Perrin, off York River, near the mouth.