For two evenings in December the water itself becomes the light show. Boats of every size — kayaks up to yachts — move in slow procession out from Shelter Island, through the harbor, toward the Embarcadero, while more than a hundred thousand people find a patch of shoreline to watch them pass. It has been happening since 1971, and the crews take it seriously: weeks of rigging, inflatable displays, themed structures built to win a judged category against neighbors they will see all year. Because the route runs the length of the bay, half the waterfront is a good seat. Free viewing along Shelter Island Shoreline Park, Harbor Island and the Embarcadero fills early, so come well before the start with blankets and something hot. Two Sunday evenings, both of them worth it. Take the kid who still believes, and the adult who forgot they used to.