Daft answer - jump off boat & start swimming away ( i've seen it done)
Sensible answer - call a quick mayday ( you can always cancel it & if it goes really wrong the boys in big orange boat are on thier way - you'll probably need a tow anyway !) .If you can , turn off the electrics & get the petrol pipe off ( cut or pull hard ! ) - get the fire extingusher ( which is of course easily to hand

) try & spray it in the inlet of the cowl. If it doesnt work that way - either stand at the other end of the boat , or keep trying if it seems to be working until its out / or the cowl melts & you can get it proper ( by which time you extingusher is probably empty).
Thats my best guess - what you are supposed to do is probably something differant