The boat diesel response is explaining an OVERLOADED engine , not one simply operating at rated power. A few hundred RPM down , with NO black in the exhaust is usually fine.
Many engines can suffer from internal hot spots , where the coolant will boil inside the engine.
On many DD the rule is never run at over 200F to avoid a cracked head.210F is DISASTER!
The 3208 is not known for cracking , even with bigger turbo levels.
The Evans coolant will allow you to run a higher internal temp for better efficiency , or simply as insurance when running really hard.
I agree a EGT on both exhausts would allow you to have a better idea about the loads.
Have you thought about replacing the boat with a design built TO plane , rather than wallow,, sort of,, on top?
Might be loads cheaper to keep.