One swimmer drowned and another 27 had to be rescued as the tail end of Hurricane Gert hit Nantucket with high waves and rip currents.Nantucket police Chief William Pittman said the beaches there were “double-red flagged” yesterday — meaning no swimming was allowed. Nonetheless, between noon and 4 p.m., lifeguards had to keep going back to pull one distressed swimmer after another out of the water.