I marinised a pair of car engines for my old boat but went for the normal route of closed circuit coolant and heat exchanger, I originally made up my own water cooled manifolds/heatexchanger/header units but the welds kept cracking so in the end went for bowman header/exchanger and separate cooled manifolds, they run smooth and saved me a fortune (about £2.5K total for 2 engines sourced, overhauled, marinised, fitted, with new iroko bearers glassed in. Ok it didn't include the silly number of my hours put into it and a few favours were used.
But with all that cold water under the boat why would you use air for cooling?