That's only 6 nights, so if the dates are right, I assume its a points reservation, not a weeks-based reservation. If that is right, you can still cancel with Marriott, but you will be limited to your use of those points. You will have to make a reservation within 60 days of travel, so likely only low season availability. You could also try to rent at a reasonably low price, to recoup some of the MF cost. There are a few pretty active OceanWatch groups on FB, MVC rental groups, or Redweek/Vacation Candy. This late in the game, you'll probably need to rent at a really good price, so a place like Vacation Candy might make it more difficult, as they add $500 to your price.