You could try contacting all of the sailing companies to see whether they can accommodate a one-way south when you want it. They might have a request for a one-way in the other direction just before or just after! I think (tho' I'm no expert) that only TMM operates out of SP now - all the others have relocated to Placencia.
Partially correct, TMM is the only Bareboat operation based in SP. Rick runs captained charters as does La Gaviota. For TMM there is a $600US drop off fee for a one way to cover the cost of fuel and staff on the way back.
Thank you Simon - I knew my knowledge was patchy, but it's still better than that of the OP. They wouldn't be allowed to captain their own boat anyway, would they? Too many reefs to hit. They should add Rick and La Gaviota to their list of firms to contact.
No they couldn't captain their own boat unless they could show a sailing resume with sufficent experience. The chart briefings for the bareboat charters are very in depth and all "no go" areas are clearly marked on the provided charts so hitting the reef is an very unusual occurence.