It's usually cheaper to buy such a ticket than to buy separate tickets to achieve the same end. After widdling down the selection of ferry services, contact the remaining few. Begin each inquiry by explaining to the ferry representative your planned date or dates of ferry travel. Ask about the sailing schedule for that day or days. Ask if the different departures during the day charge different fares.
Ferries tend to service relatively short routes many times a day.
On the other hand, ferries may service relatively long routes only once or twice a week. If you definitely need a two-way ticket, be sure to ask for return fares. If not, ask how the company calculates return fares. If you're arranging passage for a large group, ask about special group rates. Describe the height and length of your vehicle and, if it's a large van or minibus, the number of seats it has.