Very common problem. The impeller is the first thing you'll want to check. The impeller may look perfect, but I've actually had one where the brass hub came unglued from the rubber impeller so the hub spun but the impeller didn't move. Be aware that the wear inside the water pump housing can reduce the pump's efficiency, so you may still have water spitting out the back but just not enough. Check to see that the walls inside the pump are not scored or worn. Inside the housing there is a small replaceable cam that compresses the impeller blades, if that is worn down you lose efficiency. Another big thing to check is the sea water strainer. If it is not air tight, it will leak air into the system. The old Perko strainer fastens with wing nuts on two sides, if you get it tightned down at an angle, it'll break vacuum and pull air. And check the gasket in the strainer cap. If its cracked, you'll lose the vacuum and pull in air. Finally, pull off some water hoses and make sure the water passages are not plugged with scale.
I replaced the sea water strainer and the water pump. Problem fixed.
