If travelling to San Juan Island during July & August, plan to be in the ferry line at least 2 hours prior to departure. The am ferries are the less travelled. Ferry schedules change in later March for Spring Schedule, later June for Summer and later September for Fall. Spring, Summer and Fall Schedules USUALLY have 2:00pm, 4:30pm, 8:20pm departures (again these are approximates) that are the most popular. There is also an 8:25pm (and later in the summer).
There is not much food service on the ferries. The Anacortes terminal has the "Cheesecake Cafe" with a very limited menu and hours of service. We usually always eat before we get in line or bring take out to the ferry. You will notice the "locals" on the ferry. They are the ones carrying a pillow and sometimes a blanket. We also carry pillows and blankets in our car at all times. Of course, a Costco run is also mandatory when we go 'off island' and a gas fill-up. The grocery stores on the island can be very expensive (we have two but both owned by the same person).
Please remember to fill up your car before coming to the Island. Gas prices are about $.50 to $1.00 per gallon higher.