Hi GSightseeing and welcome to Google Earth!

I very much doubt that it is a speedboat going in circles. The waves are concentric. Have a look at the actual speedboat 500 yards to the northwest and then imagine it going in a circle. The pattern would not look like this one would it.

I still think this is an amazing find. The idea of it being an explosion as has been suggested to clear debris etc makes a lot of sense. However in that case would one not expect a very powerful 'lead' wave followed by the wake we see? Perhaps the lead wave has dissipated already? But then I would expect to see more interference from its reflection (similar to the reflection we see just northeast of the center.

Looking forward to more theories...