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Wikipedia has a pretty good article on it, which echoes everything I've ever been told on the subject:

http://en.wikipedia.org/wiki/Radial_engine

A relevant portion:

The radial was popular largely due to its simplicity, and most navy air arms had dedicated themselves to it because of its improved reliability for over-water flights and better power/weight ratio for aircraft carrier takeoffs. Being liquid-cooled, inline engines require the added weight and complexity of cooling systems and are generally more vulnerable to battle damage. Damage to an inline engine could result in a loss of coolant and consequent engine seizure, while an air-cooled radial could take damage but continue to operate.[7] Additionally, radials offered higher mechanical efficiency than inline engines, as they had shorter and stiffer crankshafts, a five-cylinder radial needing only two crankshaft bearings as opposed to the seven required for a six-cylinder inline engine. The shorter crankshaft also produced less vibration and hence higher reliability. Another advantage of the air-cooled radial is that all cylinders receive equal cooling airflow, and most radial-engined aircraft designed since the 1930s were fitted with NACA cowlings to further improve cooling and reduce drag. Also, by being flat, radial engines resulted in shorter aircraft with better landing visibility (very important for carrier landings), and with a smaller moment of inertia that were able to turn more tightly.[citation needed] The latter was especially important in fighter aircraft in general.

The inline engine's major advantage was a smaller frontal area compared to radial engines. This made it possible to build more streamlined designs and, for single-engine aircraft, could improve the pilot's forward visibility. In addition, being liquid-cooled offered greater options for both engine and radiator placement. For example, the P-39 Airacobra mounted the engine behind the pilot to allow the large M4 cannon to be mounted in the front of the aircraft, while the Spitfire incorporated an underwing radiator design which offset cooling drag by using the cooling air to generate thrust.[8]
 
I always assumed it was just because the radial engines were air-cooled. Not a lot of fresh water in the middle of the ocean.

Over land in the Pacific and in China where fresh water was available, water cooled Mustangs and P40 Tomahawks (the Flying Tigers) etc were used.
 
I always assumed it was just because the radial engines were air-cooled. Not a lot of fresh water in the middle of the ocean.

Over land in the Pacific and in China where fresh water was available, water cooled Mustangs and P40 Tomahawks (the Flying Tigers) etc were used.

Water in a water cooled engine is recycled is it not?
 
Water in a water cooled engine is recycled is it not?

My assumption never got that far into the technicalities sorry! I've always assumed a radiator is the weak point of any engine when it comes to bullets and flak etc and springing a leak. Like those old gangster cars blowing steam when they get hit by the Bonnie & Clyde machine guns.
 
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