Getting around Myrtle Beach without a car is tough. The Grand Strand is spread out, so you'll be limited in what you can see and do. Taxi's are available but pricey. I recommend a rental car.
Weather is usually pleasant and the ocean still cool.
Average air and water temps
If you'll drop in and ask your questions on the
Myrtle Beach forum in TripAdvisor.com, you'll gets tons of advise and suggestions. Those forums are designed to help tourists, and the MB forum gets an awfully lot of visitors, and responses will come quickly.