Just to bring this to closure. I disassembled the heat exchanger, I never did find the last 2 blades of the impeller, so I am assuming they ended up as grit and dust in the original raw water pump. I cleaned the heat exchanger (the best I could, I will try the vinegar at some point), and reassembled with new o-rings, seals and hoses (just because it was time). Opened up the end of the heat exchanger looks like this;
Initially I had a frozen bolt, I used Blaster 16-PB, left it overnight and that did the trick. The engine coolant outflow nozzle was pretty rusted, but still thick enough, so I cleaned it and used Permatex Ultra-Copper as a coating before I put the new hose on it.
After reassembly I ran the engine thru its passes and found no leaks and water flowing fine out the exhaust. This is my first boat so I am learning as I go, it is satisfying to get things back together and have them work the first time. A bit of time (and money) is all it took!
thanks for all the input.