Two ways onto an island, and often only one of them has occurred to you. The ferry terminal might be a ten-minute walk from the harbour wall. The airport might be a converted field with a windsock and a very small departures room. Or the flight is the only scheduled service, and the boat stopped running years ago.

Where both exist, the choice comes down to five things: cost, reliability, luggage, carbon and timetable. Work through them in that order and the decision usually makes itself.

Cost: price the whole journey, not the headline fare

Foot passenger ferry fares are often the cheapest way onto an island, and on many routes a return costs barely more than two singles. Flights can look competitive until you add a hold bag, a cabin bag you didn't realise was chargeable, and the seat you'd rather not sit in.

The mistake is comparing ticket to ticket. Compare door to door. Add the bus to the port or the long-stay car park near the airport; add a hotel night if the only sailing leaves at 06:00 and you live three hours away. Ports tend to sit in the town you're leaving from, which is convenient. Island airports are sometimes in the middle of nowhere, which is not.

Taking a car changes the sums completely. Vehicle spaces are limited, expensive in high season and need booking early. On a small island with single-track roads and tight parking, a car can be more trouble than it's worth — but on a sparsely served island it's the difference between seeing the place and sitting in one village. Check the bus service, bike hire and taxis before you decide.

Reliability: what actually cancels a crossing

Ferries and flights fail for different reasons, and the season matters as much as the mode.

Ferries lose to wind and swell, especially on exposed crossings and small inter-island routes with little shelter. Fog is rarely the problem. Flights lose to fog, low cloud and crosswinds, and small aircraft are sensitive to all three — a crosswind that a big jet shrugs off can ground a six-seater.

Neither option is reliably "better". It depends on the route, the boat and the time of year, so ask people who live there. A guesthouse owner will tell you without hesitation which one they'd trust in November.

What you control is slack. Don't book a ferry arriving at 14:00 and a flight leaving at 15:30 on separate tickets; if the boat is late, you've bought the flight twice. Where a connection matters, travel the day before and stay overnight. It costs less than a missed wedding. Remember that disruption hits the return leg just as often, and a February weather system can sit over an island for three days.

Luggage, kit and creatures

This is where the two options separate most sharply.

• Ferries: foot passengers usually carry their own bags, with no strict weight limit, no liquid restrictions and no fee for a second rucksack. Bikes, kayaks, paddleboards and walking poles are commonly accepted, sometimes for a small charge — book them.
• Flights: hard weight limits, and they bite hardest on small aircraft where every kilo is counted. Sports equipment must be declared and booked in advance, and excess baggage fees add up quickly.
• Dogs: often welcome on ferries, with rules about where they stay. On flights, small island services range from "in a carrier under the seat" to "not at all". Check before you book, not after.
• Practical bit: soft bags squeeze into ferry racks, overheads and tiny aircraft holds far better than hard cases. Wheels are useless on cobbles and gravel.

If you're bringing supplies — a crate of shopping, a birthday cake, a bicycle, a travel cot — the ferry is almost always the answer.

Carbon: an honest comparison

On the same route, flying generally produces more carbon dioxide per passenger than a ferry crossing. That's the simple version, and the detail matters. A foot passenger on a large ferry sharing the crossing with hundreds of others has a very low footprint; a near-empty sailing or a small aircraft is far less efficient per person.

Ferries are not zero-emission, whatever the brochure implies. They burn fuel. But weighing a short flight against a crossing of similar length, the boat is usually the lighter choice — particularly if you travel on foot or fill every seat in the car. Choosing the direct route over a connecting flight helps more than most people expect, and the slower sailing is often cheaper too.

Timetables and the shape of your day

Ferries usually offer several sailings a day, and on many routes foot passenger tickets can be amended for a modest fee or free. That flexibility is worth real money when plans shift. Flights to smaller islands are often once or twice daily, sometimes less in winter, and a cancellation can mean tomorrow rather than this afternoon.

The crossing is also part of the trip. Two hours on deck with a coffee and the coastline coming into focus is a different experience from twenty minutes with your knees against the seat in front. Neither is wrong; they are different holidays.

Watch the seasons. Timetables that look generous in July can thin out dramatically in October, and some routes stop altogether. In the shoulder months, build your dates around the transport rather than the other way round.

How to decide, in order

1. Check which options actually run on your dates, in that season.
2. Price the whole journey: fares, bags, parking, taxis, any overnight stop.
3. Count people, bags, bikes and dogs. Small aircraft punish optimism.
4. Add slack for weather, and never rely on a tight connection between separate tickets.
5. If time is short, fly; if cost, luggage or flexibility matter more, take the boat.
6. Book early for peak weeks — vehicle spaces and small aircraft seats both sell out.

Whichever you choose, know what protection you have if things go wrong. Separate tickets usually mean a delay is your problem rather than the operator's, so read your travel insurance properly before relying on a same-day connection, and ask the insurer or a broker if the wording is unclear.

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