Well I think now we know why your pump wouldn't pickup from the garden hose. When you get it all put back together, put some kind of lubricant on the impeller, close the valve on the front pickup
, hook up your muffs as before, and try again. Hard for the r/w pump to pull any suction when the front valve is open...
I'm betting that the previous owner had some run-ins with a few mud banks and clogged up the foot pickup so that his engine
overheated, so when he installed the freshwater system he provided two pickups.
Can't say I like the idea, seems more like double the chance for screwups, rather that insurance
against clogged intakes.
Would be good to know what outdrive you have, which would give us a better idea as to what advice to offer.
From the picture you supplied, the Mercruiser part number for the impeller you need is likely 47-59362T6, the Sierra (an aftermarket supplier) p/n is 18-3224, and the 'run-dry' impeller by Globe Rubber is 2001. Some people have pump wear problems with the globe impellers; I've used them without problems but they're expensive.
Your impeller looks fine to me, if there isn't any rubber rubbed off on the housing, it's probably good.