“There appears to be the same volume of water coming out of the exhaust exit as usual but it's hard to be certain.”
This actually means nothing in the MD7A. The cooling
system moves from the pump to the hard pipe that goes two places. One is into the exhaust manifold and from there into the head
and block then back into the exhaust manifold at the thermostat then into the thermostat housing and out to the exhaust. The other section of the pipe from the water pump is just a bypass that feeds directly to the thermostat housing (behind the thermostat) and out the top to the exhaust. It is possible the feed into the block is clogged and if that happens the pump will just keep pumping through the bypass and into the exhaust. With my fresh water conversion I’m actually reducing the flow through the bypass as it is not needed to keep the exhaust cool, that will be done by the new raw water
“Might it be necessary to install another raw water pump
to boost the cooling water through engine?”
No, you are getting clogged up. The passages are small and clog pretty easily. Once you start overheating it gets worse and worse quickly as you get above the point where the salt
starts crystalizing and it clogs more.
“As for air would the fuel system not need to be bled to restart the engine?”
Yes, the MD7A isn’t self bleeding. I have/had a small leak in #2 injector and once it gets air in it it will not fire/run until bled again.