According to the local news this morning, there was a really strong rip tide and the swiimers could not get back to shore. As nobody seemed to know exactly how many swimmers went in, the helicopter swept the area for about half an hour after the rescues to ensure none had been missed. There appeared to be more than 20 people in the water because people on the shore went in to try and help those already in the water.
All's well that ends well and hopefully they will have more sense in future!
Cheers, Lucy