There should be valves at the hull...make sure they are open, then try rodding out the line with a stiff piece of wire if the run is straight enough. There is a possibility that the valves, if present, will not move - but are probably open and can be cleared.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)