318 fresh water cooled overheat

Hi I have a 1961 fiberglass wheeler sea skiff I am restoring with a 318. I just put her in the water and The block is overheating. The raw water side of things is good. Plenty of water from intake to impeller through heat exchanger and out exhaust but the block is overheating. The risers are as cool as could be. Does sea water go through the manifolds with a freshwater cooled engine?The hose that goes from the heatexchanger to the thermostat has no pressure.... If the thermostat is stuck shut would there be pressure in the hose?? I plan on doing the following but any advise or info that might help me save some time would be appreciated.
1. Remove thermostat. Run engine hope that solves issue.
2. Remove Heat exchanger inspect run engine hope that solves problem
3. Remove and replace circulating pump? Is that a easy part to find? Hope that solves problem.

Does sea water go through the manifolds with a freshwater cooled engine?

Yes.

The hose that goes from the heatexchanger to the thermostat has no pressure.... If the thermostat is stuck shut would there be pressure in the hose??

Yes.

A couple of questions: Is this one of those dopey dual pocket raw water/ circulating pump deals (with FOUR hoses at the pump)?

If no--just two hoses at the raw water pump--then you might have a bad circulating pump. Not cheap but readily available.

Jeff

Confirm you are really overheating with an IR temp gun. You could simply have a bad sender or gauge.

-JJ