Loads of good advice here this is a great thread. There are several possible sources of problems here. yes you could need an impeller change which is easy enough just do remember to lub the new impeller before you install it. The inlet and outlet hoses can be blocked with growth even inside the boat. I have had that problem happen where it almost completely blocked hoses. The outside strainer on the thru hull
could be blocked. and lastly you could have growth inside the actual heat exchanger
coils on the unit. I would first confirm that the ubit is properly gassed up, then start at the inlet and verify each step the water
takes. and good luck I hope it is a crab that has taken up house keeping in the strainer.