No two ferry crossings ask the same thing of a cat, so preparation starts with questions, not purchases. The arrangement you are given decides where she spends the crossing, who can reach her and what her carrier must contain for hours without you.
Verify the Crossing Conditions Before You Book
Contact the operator for the exact sailing and get the answers in writing where you can. Ask whether cats in cars stay in the vehicle for the whole crossing, whether an onboard kennel must be booked and what size it is, and whether any pet-friendly cabins allow a carrier inside and need reserving ahead. Ask, too, whether owners may visit their pets during the crossing and at what times, how early vehicles carrying pets must check in, whether she stays in the car or comes with you during boarding and disembarkation, and what the vehicle deck conditions are like on a long sailing. If the operator's arrangement is one she would find very hard, such as a long crossing in a vehicle with no visiting access on a hot day, that is a reason to choose a different sailing, route or travel day, discovered now rather than at the port.
Once you know the arrangement, write a small plan for it: where the carrier goes, what goes inside it, when she last eats and uses a tray before boarding, and what happens in the first hour after you dock.
Build a Self-Contained Carrier for a Long Crossing
Prepare the carrier as if you will not be able to reach her, even if visits turn out to be allowed. Line it with familiar bedding that smells of home over an absorbent layer, so a spilled drink or a toilet accident does not leave her lying in the wet. For a long crossing, use a carrier large enough to include a small litter option at one end, separated from her bedding, if the verified arrangement and carrier rules allow that size; for shorter crossings, give her a litter opportunity in the parked car, doors shut, before check-in instead.
Add a cover that can dim light and movement without blocking airflow, secure the carrier door, and label the outside with her name, your name, your phone number and the words saying she is inside. Feed her lightly several hours before boarding rather than giving a full meal at the port, offer water before you load, and carry her food, spare bedding, bags and cleaning wipes in your own bag for the arrival end, not locked away with luggage you cannot reach.
If she will use an onboard kennel, bring her bedding to place inside it so the strange pen smells of home, and ask whether that bedding stays with her. If she will ride in a cabin with you, keep her in the carrier in public areas and plan the cabin corner where it will sit, away from the door.
Board, Settle and Disembark in a Closed Sequence
Arrive with time in hand so the carrier is never carried at a run through a noisy terminal. Keep her covered, speak quietly, and follow the operator's directions for her arrangement without opening the carrier in open or crowded spaces, where a startled cat could bolt across a deck or car park. If visits are permitted, use them to check her posture, top up water if the setup allows and speak to her briefly; resist taking her out for a cuddle, because returning her to the carrier mid-crossing is usually harder than leaving her settled.
If visits are not permitted, prepare the arrival end instead: water, food and a quiet first stop for a tray inside the closed car. On arrival, carry her level, load her into the car before loose luggage, and check her breathing, bedding and appetite once you are somewhere calm. The friend or family member you have named to meet the boat holds Care Pass access through KinsLe for that day alone, seeing only the crossing-day details you chose to share.