Wellfleet. Water Mill. Montauk. Provincetown. In the middle of the last century, east coast artists and writers spent their summers in cheap rentals at the shore, churning out immortal works amid ocean breezes. That’s all gone now, but for a glimpse of what was possible—in this case, what the abstract artist Helen Frankenthaler did on her summer vacation—come to the Parrish in Water Mill. —L.J.