[ndutton]

Ernst, my approach would be to start at the source (thru-hull), confirm that it is clear and shipping water, then follow the path until it stops. Nothing is good until it tests good.

As for the pump cavity, the way I've checked mine (before I replaced it with a MMI pump) was to measure the width of the impeller and the depth of the cavity with a dial caliper. You want the impeller to be a few thousandths wider than the cavity is deep so when you install the cover plate the edges of the impeller vanes are slightly 'crushed'. Of course, you'll want a good impeller and cover gasket. Shopping bags and dollar bills will work but should be considered temporary measures.

If nothing is apparent, I'd get out the pressure gauge/Schrader valve apparatus (available at Home Depot for cheap, see picture) and compressor or hand pump to test portions of the system. For example, you can remove and plug the hose from the thru-hull, attach the gauge/valve to the outlet of the pump and charge it to say, 10 psi. See if it holds pressure. If not, a squirt bottle with diluted dish soap sprayed on everything will tell you in an instant where the leak is. It'll foam up like a 5 year old blowing bubbles.

Finally, if my boat were a 2 hour drive away I'd load up the truck with everything I'd need and a few extras for all the tests mentioned and I wouldn't leave until the culprit was found. Hell, I'd do that if my boat were 15 minutes away. Anything I didn't have with me, I'd go buy.