That's a good point. When the oil cooler on my engine leaked several years ago a couple of the cooling hoses deteriorated and failed within about a month of the leak. The heat exchanger was a mess. I had to pull it and clean the oily sludge from it. The raw water pump impeller wasn't affected because the oil leaked into the fresh water cooling, rather than the sea water cooling. I think the impeller is probably impervious to oil. Still, the entire raw water side of the system - including the aftercooler, raw water elbow, and exhaust tubes could become saturated. I was surprised how fast and to what extent the hoses deteriorated. Despite all the cleaner I put through the cooling system, the oil clung to the hose like velcro.
My previous boat, also raw water cooled, managed to survive a large oil spill in Portland Harbor without any apparent ill effects. If I were operating in the area of the spill on a regular basis I would pay special attention to the cooling hoses - if nothing else.