Hello fletchccc,

Neither GND or SLU will issue an outbound clearance like that for a traveler. Remember, the outbound clearance that you do on a boat has a full crew list, for the Zarpe, and you'll have to show passports for each crew member upon clearing out. You cannot "split" clearing out between the airport and a marine port-of-entry. Sorry.

But if you are heading north from Grenada you will probably stop in Carriacou and stay in Tyrell Bay (or perhaps Sandy Island), so you can clear out there. Currently "Zanshin" is waiting for me there.
Likewise in St. Lucia you'll most likely be heading north, so stopping in Rodney Bay and dinghying into the marina to clear out is done very quickly. The office is fast and efficient, I kept my boat there for a couple of years.