>>Tennis court
Looks like canvas camouflage with tie downs.
Look at the house at the north end of the island in 3D mode and it looks like a "tarp" covering a hill, too.
Go here and experiment for yourselves. To change the viewing angle, hold the CTRL key and drag the mouse pointer up and down. Use the scroll wheel to zoom in and out:
Google Maps Little St. James Island
There are other weird things going on at that island. Look at the beach south of the tennis court. It looks like the entire beach area has been blocked out for privacy. Look at the area just to the south of the boat dock. There seems to be a giant warehouse and an outdoor amphitheater. In the center of the island just to the east of the tennis court appears to be gigantic sundial. On the north end of the island there is a large compass rose. And of course in the southwest tip of the island is the golden domed 'temple.'
Weird stuff, but other than why put a tennis court so far away from the main house, there isn't a tennis court-shaped tarp on the island.