Fourteen degrees Celsius water temp. was the magic figure in my days in the RN. Below that, and dry suits were mandatory for flights over water, and that is borne out by the graph above.
I'd also argue that anyone who is regularly flying over water in a single-engined aircraft should have some realistic training in ditching and escape from the aircraft. My 16 y/o daughter recently did the
Dunker - fun it isn't!