I would close the raw water intake thru-hull and spin the top off the raw water strainer housing, and run a garden hose into the strainer housing until it fills, then start the engine. No pressure at all. The water pump would pull in the water just as fast as the hose can feed it. It might be slightly water starved but it would be enough to keep water circulating at idle. Can't post a picture right now but my strainer housing looks like this