On a coastline broken by water, the practical length of a day's walk is set by boat departures rather than by fitness. The timetable is the route plan.
The last sailing is the hard deadline
Missing a final ferry does not mean a late arrival. It means being on the wrong side of water with no alternative until the next day.
Coastal walking routes are therefore planned backward from that departure, with the turnaround point chosen to leave a real margin.
Daylight, which governs inland walking, is usually the looser constraint, since ferry services typically stop well before dark outside the peak season.
Frequency collapses outside the season
Many coastal and island services run a summer timetable with several daily sailings and a winter one with a fraction of that.
Some routes that carry walkers exist only in the summer months and are withdrawn entirely, removing a link that a route description assumes.
Route guides written for peak season can therefore describe a perfectly reasonable day that is impossible in the shoulder months.
Weather cancels boats, not trails
A ferry crossing exposed water has a sea state limit, and services are canceled on conditions that would not stop anyone walking.
Cancellation decisions are often made a few hours ahead, which is after a walker has already committed to the far side.
Experienced coastal walkers treat the outbound crossing as the decision point and check the forecast for the return before boarding.
Tide affects some landings
Small piers and slipways on shallow coasts can only be used within a tidal window, so sailing times shift day to day rather than repeating.
A timetable that lists different departure times each day is usually tide-driven, and reading last week's times is a reliable way to miss a boat.
Where a landing is by small boat onto a beach, the same swell limit that governs a cruise ship's tender operation applies at a smaller scale.
Building a route around the constraint
The most robust plan uses the ferry outbound in the morning and walks back to a point reachable on foot, removing the return crossing from the critical path.
Where both legs must be by boat, the useful margin is at least one sailing rather than a fixed number of minutes.
Carrying enough to sit out an unplanned night is standard practice on routes where a cancellation strands walkers, and it is a lighter burden than the alternative.