In Venice, a city famous for being visited by too many and home to too few, children’s play now fills neighborhood squares, fishermen sell their catch to home cooks, and water buses convey masked and gloved commuters to businesses preparing to reopen. At the same time, the famed lacquered black gondolas remain moored to the quay; hotel rooms are empty, museum doors sealed; and St. Mark’s Square — normally teeming in any season — is traversed at any given moment by just a handful of souls after tourists abandoned the city in late February. For years, Venice has faced an almost existential crisis, as the unbridled success of its tourism industry threatened to ruin the things that have drawn visitors for centuries.