Yes, I can confirm that the Crescent outboard engine, that was used in some motorbikes, was water cooled. I worked in the firm that constructed it. It was a three cylinders two stroke 500cc. There were two models for motorboat racing. One on gasoline and one on methanol for the classes CS and C. The Swede A V Andersson raced his Crescent solo bike in the Senior TT 1969 and he was a DNF.