Manifolds and risers are nice and clean! (make sure you check for "flat" and replace riser gaskets!) If those are the original risers and manifolds, I would expect the block and heads to look the same.....
Raw water impeller is just fine.
I would want to check carefully for bubbles and flow-rate to the stat housing to eliminate it as a problem.
Also, pulling your risers was a good idea since it looks like there might be the start of riser gasket leaks. That's particularly bad because the leaking occurs right after shutdown. It allows a small amount of water to run into the exhaust and into any open exhaust valves, causing a hydrolock during the next start up if you try to start it right away. Once you get it running, any water is "blown" out with the exhaust......
If you don't start right away, the little bit of water will eventually get past the rings and end up in the oil. Not a huge problem since it does evaporate rather quickly once the engine gets up to operating temp.
The main problem comes when you park it for the winter with water in the cyl that had the open ex-valve......... then you get rust if it's wet in there for the winter!
If you have good flow with no bubbles, your stat is good and the circulating pump is good I would consider looking for sand in the block....
I think you could remove both side core plugs and flush with a hose and probably get most of it....
