318 heating up at around 2500 rpm

My portside engine was running

My portside engine was running up around 200 degrees and climbing at about 2500 rpm yesterday. When I slowed and stayed around 1900-2000 RPM she cooled back down. The starboard engine stays pretty consistant at 160 degrees. When at idle the gauge reading for the port engine was below 100!? I am thinking the gauge may be the first thing that is causing problems? If not that then themostat. This is a 1971 BurnsCraft Houseboat with Twin 318's and Dana's. I am not sure how to tell the difference in what type of system I have. I am familiar with automotive cooling systems. This have a typical type waterpump?

Get a laser-pointed temp gage

Get a laser-pointed temp gage and see what's happening instead of guessing. About 30 bucks at Harbor Freight.

JEff

Sounds like a clogged intake t

Sounds like a clogged intake to me.
The cooling system works alot like a car but you might have a raw water heat exchanger mounted on the back of the enginge in which case you would have another pump too.