Ferry / Cruise travel

For island-hopping in Greece or crossing between countries separated by water, the ferry isn't really competing with the other four — it's often the only surface option, and sometimes the more scenic one even when flying is available.

Pros

Only practical surface link between islands and across certain straits
Can carry a car, useful for road trips that need to cross water
Scenic — arriving by sea gives a different first impression of a coastal city

Cons

Weather-dependent, delays and cancellations happen
Slow compared to flying between the same two points
Seasonal schedules — routes shrink or vanish outside peak season

Best use cases

Greek island hopping and other archipelago trips
Crossing straits where no bridge or tunnel exists
Road trips that need to bring the car along

Environmental note

Varies enormously by vessel and speed — a slow conventional ferry is efficient per passenger, a high-speed catamaran burns closer to what a plane does over the same distance.

Speed: Slow to moderate, faster catamarans exist at a cost
Cost: Low to mid, rises for cabins on overnight routes
Comfort: Varies — deck seating is basic, private cabins on overnight routes are comfortable
CO2: Low on conventional ferries, higher on high-speed services