I don't have a cut off or seacock that I can find.

What sort of boat is this that it doesn't have a cut-off for the raw water
intake? Yeesh, that's crazy.

Also consider an electric fuel pump. If the replacement of the mechanical
one is prohibitively expensive (or difficult) you may be able to get by with
an electric one. If only until you get the boat out of the water so you can
replace the mechanical one properly. If you can find out the proper flow
rate you ought to be able to use an automotive electric fuel pump to handle
it.

-Bill Kearney