While my yacht is on the hardstand I run the engine every couple of months for about 20 minutes. Since it is raw water cooled I use a fresh water hose to feed the raw water filter which in turn supplies the engine. The water flowing through the anti syphon system is still a good flow.
Usually I notice the wet exhaust spitting out water from time to time but the last two times there has been only one or two "spits" of water from the wet exhaust during the 20 minute run. Today I also noticed I could only place my hand on the rubber exhaust pipe that is attached to the mixing elbow for 5 seconds before it was too hot for me. I don't recall the rubber exhaust ever getting so hot but I could be wrong.
I suspect a problem so my questions are:
(1) Should the rubber exhaust hose downline from the mixing elbow only ever be warm to the touch?
(2) any suggestions as to where I can find online a diagram of the internal construction of the exhaust manifold and mixing elbow?

I would start by pulling the hose off after the water pump and start the engine to check whether the pump is working. You could also just put your finger on the face plate while the engine is running and check that it is cold. The cooling passages in the block sound like they are blocked with calcium. Not enough water is getting to the exhaust elbow, the rubber should only get warm.
Any diagram from a raw water cooled motor will be pretty much the same as what you have. Because the engine starts and runs well the exhaust is unlikely to be blocked. The problem will be in the passages.
Spend sometime watching a few Barnacle buster videos. Most of them are for heat exchanger engines but it works the same as raw water cooled engines. You don't have to use the Barnacle buster product of course, there are plenty of cheaper alternatives.

-sfqZobO8mP

Presumably the hose on the left comes from the water pump. I would suggest pulling the hose off the pump end and pull the hose off up at the siphon. Blow the water out, it will taste like crap! Raise both hose ends and secure above the cylinder head height. Fill the system with a mixture of phosphoric acid of your choice. I use Metal gleam at the recommended mixture or you can use CLR used at a strong mixture. Only need about 2 litres for two lots. Let that soak for an hour or so before draining it out and putting in a second lot. After that just re install the hoses and run the engine with water running through the system. It takes very little calcium build up to get the symptoms you have.
I'm making up a pump system with a $20 diaphragm pump so I can circulate the acid for an hour or so and do that once a year.

I only know enough to be dangerous! I presume you are trying to heat water using your engine. I'm sure there would be youtube videos covering this. I only use my engine for relatively short periods and don't have much interest in a heater. I would suggest inquiring on one of the more cruiser oriented yacht forums.

forums.ybw.com/index.php?forums/practical-boat-owners-reader-to-reader.13/

www.cruisersforum.com/forums/