If you are going through Interval, and want to stay at a Marriott, then you will need to trade your week. You will need to reserve a Cypress Harbour week through a Marriott Vacation Ownership Advisor and then deposit that week with Interval. There are several Marriotts on Hilton Head (six?) and summer is the highest demand season, so it may be a tough trade for 2016. 2017 may be a higher likelihood because you will have the full 12/13 months before check-in to try and match your trade, but at this point, many people have been trading for 5 months.
Good luck and congrats on your purchase!
Best,
Greg