As jimbo said, the 1950's-1960's era 35-40hp Big Twins were particulary prone to runaway when revving in the driveway. But they also were fitted with a vacuum cut-out switch which would slow them down (if it was working) Anything over 2000-ish was likely to initiate a runaway on those motors. I've initiated the runaway many times on those when working on the DC generator systems. You had to get them up to around 2000 before the generator cutout relay would close. So, you get the picture. I once had a 40hp Scott-Atwater go berserk and wouldn't slow down. No vacuum switch on those. I wound up grabbing a tool and busting a spark plug (in terror). So, bottom line, is it depends on the particular model. Some are much worse than others.
As for the comment about the guy not knowing what he is doing, if anybody reads these forums very long, he is going to see a topic where somebody says "It will run full wide open on the hose, but has no power in the water". Wide open throttle on the hose?? Gimme a break.