I am wondering why you can't use a aluminum engine such as a 3.5 chrysler with a electric fan to the rad. and forget all this raw water & manifolds etc. surely you can get enough air to the rad by ducting or raising rad. to catch air.?
You may want to consider the ambient heat generated by an engine in an enclosed space. Even the most cramped car engine compartment still exchanges air through the bottom of the car.
An engine in a boat sits in a "bowl".
Even on the hottest days, raw water is far cooler than engine coolant at temperature. That's why marine engines with closed cooling systems can get away with using a small exchanger. It also allows open systems to cool efficiently.
All the expensive marine parts in a boat engine are there to protect against radiated heat from the exhaust manifolds as well as cooling the engine.
Also water can remove much more heat than air, ie it takes much more heat to warm a litre of water than air. In the old days many engines were air cooled, but changed to water or liquid cooling because it is more effecient. You would need a huge radiator and a very large air flow to make it work. It would probably be easier to try and rig a water cooling system onto your engine than install a rad.
