Hey Bartow, glad to have you on the site. You don't need much in the way of skills to keep these boats running, there's just a couple of quirks.
My boat ran at 171 deg when brand new, and at 167 since I changed the impeller (once in 601.7 hours

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Temperature is a big issue on an aluminum engine, you have covered the basics with a new thermostat and impeller. You could also replace the temp sensor on the block, I'm not sure where it is but I'll check. It is probably the same as an LS 400 part, probably pretty cheap. I'll look in my 1UZ engine manual and get back to you.
I would definitely look at the sea strainer and make sure that it is not full (easy to do yourself) and that there are no obstructions in the cooling system. With the engine running and a Fake A Lake (or hose jammed in your water inlet) you should have water flowing out both sides of your exhaust flappers at the back of the boat. It kind of comes out in surges, but there should be good flow.
If you don't have good flow or want to check it, I would remove your impeller, put the cover back on the water pump and hook it up, and put a hose in the water inlet and turn it on full blast. That should flush any crap out of your water jacket. It is easier than it sounds, there are pics on the HOw TO page.
It is quite possible that your gauge is not accurate. The gauges are driven off a Borg Warner computer box that is a common ski boat part. They suck on a Malibu too!
You might find someone with an infrared temperature gun, shoot the thermostat housing or intake when the engine is good and warmed up to verify the dash gauge. If you have the Multigauge digital display, check to see if the temp is the same on the needle gauge and the digital gauge, it is displayed on both. Both readings are coming from the BW MDDC box, so they always read the same in my experience.
Anyway, glad to have you on the site, there is a lot of info here.
Pete