Well it looks like you engine has the warm manifold set up where the manifolds don’t get cooling water till the thermostat opens; the elbows get cooling water all the time. They are fed by separate hoses.
In the diagram they show the cool manifolds set up where the manifolds & elbows are fed by only one hose & get cooling water all the time.
Merc went to the warm manifold set up to keep the manifold warmer than 120* or so to prevent condensation in the exhaust system.
You should test the output of the impeller with the boat in the water as a first step. If flow there is adequate then you should check to make sure the elbows are not clogging with rust.
Also did you use a Marine thermostat they will be set to open at much lower temps than automotive (140 or 160 depending on the model). I usually test them to make sure they open at the right temp. Your actual running temps should be from the rating of the thermostat to maybe 15 degrees higher at the hottest (like after coming off plane to idle speed).
