Rates are set according to how many zones you're traveling through - if you're taking a water taxi between Cherry Grove and the Pines, you're remaining within one zone (Zone 6), and so the cost is the minimum of $6 per adult. For longer trips, say from Cherry Grove to Ocean Beach, you'd be traveling four zones (which costs $12 per adult). This is the fare situation for traveling laterally, that is, east to west or west to east among the Fire Island communities. Regular water taxi service is offered daily throughout the season (late May through early September), from 9 am until 9 pm, but it's a good idea to call for the most up-to-date times.
Fire Island Water Taxi also operates a charter Cross Bay ferry service between the Long Island town of Bayshore and most of the communities on Fire Island (rates start at $125 for first six fares, and $15 per person beyond that). It's an expensive way to get across the bay, but if you miss the Sayville Ferry or simply want to travel according to your own schedule, this is a very handy service.