Volvo D-12 engines

Water in, water out. Why are you asking?

Ex-Volvo guy here.

Speed as such doesn't tell much.
Depending on the boat, you might be either at max rpm and full load, or at a relatively low rpm/load, or anywhere in between.
Anyway, full rpm/load aside, coolant should be about 10 deg lower than that, by and large.
Beware of acid washing heat exchangers, that only cleans the raw water side but the fresh water side might still be dirty.
It takes a full removal for a proper job, the rest is just corner cutting.

I run D13-1000s. Since new they run at 189-190 at cruise speed. Never seen them higher than 190, even at WOT.

And the water inlet is how big while up on the plane?

is this a new issue? When did it start? Any changes other than acid wash of the H/E’s? What temps did they run previously? What load % are you at when seeing these elevated temps? Do temps climb further with more throttle?

It's fair to expect that the builder fitted a large enough inlet, I reckon...
...and I can't think of any reason why that should have been changed/restricted afterwards.

OTOH, it would be interested to hear the answers to Dockmaster's questions.

There is this: https://www.yachtforums.com/threads...thing to do, but you're fine temperature wise...

The inlet and outlet of cooling water is a pretty basic plan for most of us.

I ran a pair of D-12 800's for many years and they always ran 183-85 at 80% load. At WOT, I would see 190, but nothing as high as you are seeing.

Use a hand held infra red gauge on the thermostat house or HE body to determine if the sensors / dash instruments are accurate .