This O/B is a 25hp & I believe that it will still work after an EMP event, as long as the gas holds out. We rarely keep more than 15gals onboard because of the flammability issue.
I think gasoline might be something a wily boatman could scrounge as he made his way down the river. If things are so serious that your boat cannot get anywhere, woudln't there have to be some stranded barges, some of which might have a little fuel in them? I know the whole industry is better at pumping out and draining barges, but it cannot be possible to really get it all out.