For island travel in Korea, a timetable is not the same thing as a confirmed sailing. The Korea Maritime Transportation Safety Authority (KOMSA) publishes passenger-ferry operation information and links to PATIS for detailed vessel and route status.
Check the route again on the day of travel before leaving for the terminal. A ferry that appears in a seasonal timetable can still be delayed, suspended or changed because of wind, waves, fog, vessel issues or port conditions. If your island has only one or two departures a day, build in enough time that a missed or canceled sailing does not automatically break a same-day rail, bus or flight connection.
Use the official operation-status page as a status check, then confirm ticketing, boarding cutoff, vehicle-loading rules and any passenger-ID requirements with the actual ferry operator or terminal. Those details vary by route and operator and should not be generalized.