Having overheating problems with a 6.0L. Was reading through old threads and came across this one that mentions on the ZR409's you are NOT supposed to have a thermostat in the "traditional" position.
Are you supposed to have a thermostat in the housing on the front of the engine just behind drivers seat?
"I have the same boat as you with a similar history. First off, I hope you didn't install a thermostat in the traditional position when you realized one wasn't there. The thermostat on that engine is back near the heat exchanger. https://www.ballofspray.com/forum#/discussion/15876/2011-nautique-200-zr409-6-0-overheat-issue
We blew a rod last year, installed a new crate engine and after getting everything attached and hooked up, the engine was running up to 200 degrees after 4-5 minutes. Pulled out the thermostat and boat ran ~160 all summer. Figured we would deal with it in the fall and try a new thermostat. Now it is fall and with the cooler water temps, we put a new thermostat in and boat runs up to 200 again.
Has new impeller and we bled out the air as best we could. May try @Jody_Seal
trick of tilting the boat up on the trailer to get the air to burp out.
But if indeed you don't need a thermostat at the front of the engine, that would explain it.