If you are in St. Lucia now, then try to
head to the bar at Rodney Bay Marina for happy hour and talk to the people there. Someone might be going, or know someone who might be planning on going.
Taking a one-way crew to Martinique is not an issue if the crew holds an EU passport. That allows the
captain to take those crewmembers off the crew list without problems. Non-EU passport holders would normally be required to prove onward-bound travel (e.g. airplane ticket) to be taken off the crew list. The French islands are generally quite lax about enforcing this
rule and most of time a
captain can get away with it. But it does present a potential problem.
Certainly upon return to St. Lucia you would need to provide proof of onward travel in the form of an airplane ticket (unless you are a St. Lucian or resident), otherwise the captain will not be able to take you off the crew list.
All this was a preamble to warning you that you might not find a
boat willing to do this. The trip is so short and many people do 1-2 day round-trip to stock up on
cheap items or items unavailable in St. Lucia. A round trip is easier, but you'd still need to show that airline ticket leaving St. Lucia.