Hi Low Country Home Builder, I used to crew tall ships for beer money and although it was a while ago the best way to find short-term positions was when they're off on a delivery or similar, maybe just out of a yard, and it's, i'm afraid, usually done the old fashioned way - walk the docks where they are, just go up to the boat directly and ask, and talk to everyone around you. Often someone will know someone who needs someone. My first place i got just by asking straight up - though i had to spend a month painting the bilge first - tough love! For internet try
ASTA in the first place.