Donald, I've seen exactly the mechanical pumps you're talking about on speed boats. That was done so if the diaphragm was leaking, it would blow the excess gas into the carb and make it run badly so you would notice it and pay immediate attention to it. Without it, it would fill the engine's oil pan with gasoline.
Oddly, I never saw it on every day run of the mill marine engines which didn't get the constant attention of the big blocks found in Cigarettes and other go fasts.
The ONLY time I ever installed an electric pump on a boat was when we were building a new V6 for a friend's bowrider and used a block we bought at the junk yard. We already had it all machined and cleaned and after we installed the crank, rods and pistons, I noticed it wasn't machined for a fuel pump pushrod. Kinda late at that point so we installed a Stewart Warner marine fuel pump with a regulator and oil pressure switch. Worked great for about fifteen years.