Rising sea levels are not only going to increasingly flood parts of Long Beach, but could leave the most vulnerable neighborhoods uninhabitable within a generation or two, according to a city presentation Monday night that drew more 300 residents concerned about the city’s — and their own — future. “Residents can continue living on Naples and the Peninsula for several decades as long as you’re willing to experience occasional flooding,” said Jerry Schubel, a marine scientist and CEO of the Aquarium of the Pacific. “(But) over the next few decades, you need to think about moving.