Disney has two beach resorts so make sure you know which has your reservation.
Disney's Vero Beach Resort - Vero Beach, Florida.
Disney's Hilton Head Island Resort - Hilton Head Island, South Carolina
The Vero Beach resort is oceanfront. The HHI resort is not. It's over a mile from the beach and has a Beach House which may be accessed by guests who bike, walk or drive there.
Both of these Disney Vacation Club resorts do offer a lot to do and we've enjoyed the HHI resort very much.
But, we've found there was
very little that would appeal to a teenager who mostly seeks out other teens socially. The 15 year old may really prefer a more active and populated area than either HHI or Vero would generally offer. JMHO.
Besides the many good suggestions from above posts, you might also consider one of these destinations as well, depending on how you are visiting the area:
Daytona Beach - newly renovated boardwalk area with some beach areas that are pedestrian only and some that are open to motorized traffic. Some of the resorts there offer organized activities for
all ages. A good location if you are driving to your vacation via I-95.
Fort Lauderdale - tons of family-friendly activities, sports, boating, sightseeing. About 4 hours drive from Orlando, not bad if you will be flying in and renting a car. Warmer weather in cooler times of the year since it's further south (like Captiva/Sanibel, though an entirely different atmosphere).
Destin/Panama City - very, very popular with families, on the panhandle/Gulf. Good stop if you are driving to FL from the west.
All of these have a variety of resorts. Many do offer children's activities daily and the whole atmosphere can vary a lot, depending on where you stay. But overall, with teens, I think they offer enough to keep them happy. Our first choice for vacationing has changed a lot depending on the ages of our kids. Virginia Beach, Myrtle Beach & Fort Lauderdale never appealed to us - until our kids reached their teens and just loved to be where all the other teens are! Eventually, when the kids are grown, we'll return to the more lush, peaceful destinations again!

HTH.