The water was only a few feet deep. Then it turned red. A startling video from Great Point on Nantucket captures a great white shark attacking a seal virtually in the shorebreak, transforming otherwise gray-green shallows into a bloody feeding zone. Carrie Drayton and Tyler O’Brien were driving toward Great Point to fish Sunday when they noticed something thrashing near the wash-over at the Coskata-Coatue Wildlife Refuge. Within moments, the water around it became a swirling red cloud. They stopped, and O’Brien began filming as the shark tore into the seal just feet from dry sand. It was a full-blown bloodbath in ankle-to-waist-deep-looking water — and another reminder that Nantucket has become prime great white territory. The reason is food. The Atlantic White Shark Conservancy says white shark abundance around Cape Cod has increased over the past decade “primarily in response to the local population recovery of gray seals,” one of the species’ favorite prey items.…