I have just had a bilge pump pushing ProFlush acid though my Yamaha - up the pickup that goes down to the leg.
It was running and has removed saltwater crud and also corrosion on the block.
I have run the engine for 5 mins without overheat warning, I will run for longer once neighbours are not all outside enjoying the weather.
https://www.force4.co.uk/item/Force-4/ProFlush-Marine-Engine-Descaler/4C1F
I run the engine on muffs at home after every use (bar a week a year over the last 8 when i have had it on a mooring), and it still crusted up. The hose connector only flushes one side of the thermostat, so that is not much use. It's worth checking the corrosiion / build up behind the stat on a regular basis, and being prepared to flush the crop once in a while.
Just been though and found my last engine flush / clean post.................2007! so 15 yrs later, not surprised she needed some love!