Does This Debunk the "Coolant Can Flow Through the Radiator Too Fast" Idea???

I've often mused how raw water cooled boats (inboards and outboards too) running in various raw water temps from 32* melting glacial lakes, oceans, as some of my boats have been, how they maintain the 140* engine running temps that raw water thermostats run, (to avoid mineral (salt) crystalizing in the cooling system), - if somehow the thermostat didn't modulate the flow ?
Was there a valve I was supposed to adjust ?