You can call the resort (843-363-3400) and ask to speak to the concierge desk. They'll be able to give you the current prices for cabana/beach chair/etc rentals and other onsite activities, and let you know when it will be possible to reserve them. (I wouldn't think you can do it prior to check-in?)
The concierges at SurfWatch are very helpful when it comes to making off-resort restaurant/activity reservations. July 4th week will be max capacity all over the island so if you have something special in mind, ask them. You'll probably be told to call two weeks in advance of your stay to confirm what you want.
Ask the concierge to also send you this week's Activities Schedule. There's no guarantee that your week's activities will exactly match but seeing a current one will answer many of your questions. If you're on Facebook "like" the official SurfWatch page - every few weeks they post an Activities Schedule.
Remember, the concierge's main objective is to get you booked for a Sales Presentation so be prepared for that when you initiate contact. It's not a trade-off, though - they're happy to help with actual concierge services even if you say no to the presentation.