If you are operating the boat at night, then the exhaust manifold might well glow "cherry red" in the dark. If you can see the manifold glowing in the daylight, then, I agree, you have a cooling problem.

The following statement is from the 2010 Briggs update book, page 54.

"We have received a few reports of large displacement engine exhaust manifolds or mufflers glowing red. In all the cases we investigated, the engines ran correctly and did not experience any performance issues. Our investigations further revealed that this is not due to a Briggs & Stratton manufacturing defect."

There was no explanation of the engine models involved. The classroom instructor stated Briggs did not want to criticize the equipment manufacturers who designed the exhaust system.

It's not unusual for a manifold on most larger engines to glow dark cherry red, especially if it is being run at full throttle. You should not see this in full daylight, but under an engine hood it could certainly be visible. If it was running lean enough to be running too hot then I would expect the usual behaviour when an engine runs too lean.

On a gasoline engine, lean mixtures will not cause the exhaust manifold to overheat(although it does kick combustion chamber temps up considerably). It's quite the opposite, lean exhaust runners run cool.

Biggest cause of overheated exhaust nanifolds is retarded timing.

Another thing to consider is that a boat engine, be it inboard or outboard is under a constant load. Unlike a car, or even a lawn mower, it never really has the opportunity to "coast," or have the load reduced (other than throttling down.) Because it is constantly working, it will run hotter. If it behaves like it is overheating, or laboring, you might consider a lower pitched propeller. You may lose some top speed on your boat, but the engine won't be working as hard.