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Im trying to understand your symptoms better. Can you please correct the following statements?

1. It runs 160 on the hose without a thermostat.

2. It runs 200 on the hose with both the new and old thermostat.

If thats the case, then clearly the problem is not thermostat related. The engine should be struggling to break 100 degrees without a stat.

Have you had the boat on the lake to see how it runs? Have you tried revving the engine to see if the temps come down? With no load on the trailer, it should be artificially easy for the engine to maintain temperature... so something is wrong. Without knowing how it acts on the water though, its a little tough to pinpoint. It would probably have to be a pretty major air leak combined with insufficient water flow for that to be the problem. I would lean towards a blockage in the system or a RWP needing a rebuild.

If you have already inspected for blockages and come up empty, I would start by testing the RWP. Dump the fake a lake and make the pump pull from a bucket instead of forcing water up to it. If it struggles to pump water, youve found your culprit.

How about your water strainer, gasket in place?

waker319 wrote: Where would you think the blockage could be at?

Possibly in the motor. What kind of water was previous owner boating in? Check all connections as well. Tighten all with a nut driver, not a screw driver. Again, hang tight help is on the way.

there may be blades of past failed impellers impeding flow...

Where all the impeller blades accounted for? Any missing? Where the blades installed in the same direction it which they came out? The real experts will likely chime in @ 4-5 a.m EST. They can likely elaborate more and give you some expert advice.

Did you have a heat problem before the maintenance? Impeller right direction?