"difference between isolated and scattered thunderstorms?"
Storms can pop up anywhere at anytime but most likely after 4:00 PM or so.
Isolated, like the storms stay on top of South Beach and it keeps pouring.
Scattered, storms popping up all over the forecast area, in this case all of Dade County and probably Broward County too.
Isolated and Scattered means there is no main weather flow such as a front making the weather. Probably just afternoon heating inland firing up T-storms and on the beach the ocean winds bumping up against the hot land air firing up T-storms. They don't last long but can be pretty violent.
Don't worry, you'll most likely get some good beach time in or you could see a Tornado on the Beach like a couple of years ago (Rare but a sight to see).