Travelers assume an island crossing can be booked close to departure. On French routes in summer the vehicle decks fill first, and they fill early.
Vehicle space is the binding constraint
A ferry sells far more passenger seats than car spaces, so a sailing can have plenty of room for foot passengers while being completely full for vehicles.
Cars, campers and trailers occupy fixed lane meters, and a camper consumes several times what a compact car does.
That is why availability searches show a crossing as sold out when only the vehicle deck is genuinely full.
School holidays concentrate demand
French school vacation dates are set nationally in staggered zones, and summer travel compresses into a predictable window around them.
Changeover weekends produce enormous peaks, since a large share of the country begins and ends its vacation on the same days.
An American traveler unaware of those dates can land on the single worst weekend of the year without realizing it.
Island supply cannot expand
The number of vessels serving a route is fixed for the season, and adding a sailing requires a ship that is already committed elsewhere.
Ports have limited berths and turnaround times, which caps daily frequency independently of how many people want to travel.
Unlike an airline that can upgauge an aircraft, an operator cannot easily add capacity to meet a surge.
Prices move with the calendar, not with the clock
Fares rise steeply for peak sailings but do not swing minute to minute the way airline pricing does, so late booking rarely produces a bargain.
Night crossings with cabins are priced as a separate product, and cabins sell out earlier than reclining seats.
Flexible tickets cost more but matter on routes where a canceled sailing due to weather leaves few alternatives for days.
Going without a car changes everything
Foot passengers face far less scarcity, and a rental car booked on the island is often cheaper than shipping one across.
Island rental fleets are small and also sell out, so the booking still needs to be made well ahead even without a ferry vehicle slot.
Corsican roads are narrow and slow, and driving times run far longer than distances suggest, which makes a smaller vehicle a better choice than the one a family would ship from the mainland. A large American-sized car is a liability on those roads rather than an advantage.
Air links to the same islands run alongside the ferries and behave like any other short flight, so a traveler willing to skip the vehicle has a second route that the ferry schedule does not constrain.