my only experience with a similar situation was on my Merc 50hp .. years ago… believe it or not… I found a grommet in the water cavity under the water pump.
it drove me nuts … my 50hp would steam it was so hot at times at idle it was fine at high speed HOT!! the grommet would rise and plug the water flow into the pump.
I have basically left the dark side now but had a 9.8, a 45 hp Classic 50 and a 500 all great motors
Joining AOMCI has priviledges 🙂