I could use some help on my buddy's boat im trying to get ready for the rest of summer. Its been running great with the exception of a few small gremlins... He has twin 575 sci's with bavo xrs with sportmaster lowers. Both motors and drives recently rebuilt. First problem is the port motor recently started running warm staying right at 190 through all rpm ranges. replaced seawater pump, cleaned strainer and back-flushed system (mind you old impeller was still intact when replaced). also changed thermostat, the housing cover does show signs of corrosion restricting water flow slightly, but wouldn't that cause a problem over time and not just the last few trips out it started getting warm.

Second gremlin is the starboard outdrive is building pressure in the bottle and blowing it out the top. When it started doing that I drained fluid, noticed it was slightly discolored, pressure tested drive with a modified cooling system tester and it only dropped form 17-15 psi over the span of 24 hours. Replaced the entire reservoir and cap assembly still having issues. and yes it has drive showers installed

I believe my old 575's ran about 150 degrees, but I had 120 degree thermostats. I'd be really concerned at 190 on a 575 (detonation).

Have you checked the water inlet side of the sea water system for restriction? What water pressure are you seeing (at idle, at cruise, at WOT)? Does it cool down at idle speed? Pumping good water from exhaust?

Check for corrosion inside the water neck on the transom assembly.