The story was strange enough to be a child’s fable: In an isolated section of the Chesapeake Bay, there was a two-story Victorian house that seemed to emerge directly from the water. And, scurrying around it, there was a retiree, trying to keep the house from falling in. Finally, the man gave up. And last week, the house did, too. Raked by a storm, it cracked at the spine and collapsed into a one-story wreck. The tale of the house and the man illustrates the Chesapeake’s problem with rising oceans and sinking land. It has already erased life on most of the bay’s islands and now is threatening to erase the islands themselves.