The lowest cost and most scenic form of transport between Greek islands, to the mainlands and to Turkey and Italy is by ferry. You need to have the time, as some of the trips can take more than 12 hours. The standard ticket is "deck" class, meaning no reserved seating and no cabin. During the non-busy time I traveled there was plenty of the coach style seating. These seats recline slightly, but are still uncomfortable to sleep in. Seasoned travelers arrive with their own bedding and sleep on the floor. For less hardy tourists there are some reserved seats more like reclining aircraft seats and there are cabins (but these cost more than an airfare).
One tip is to choose a location away from a door, as these will be opening and closing all night. Don't try to sleep near a TV, as these are on all night showing movies.
One problem is that many areas of the ships permit smoking, or allow it in an area no sealed from the non-smoking areas. However, this is made up for by the fresh air and views of passing islands from the deck.
Some of the ferries are the size of cruse liners, with escalators, bars and restaurants. The food is good, with local wines and some good local Greek cooking (much better than airline food). The ship I was on had a swimming pool (empty) and offered Internet access (broken).
One form of entertainment is watching the trucks maneuver to load onto the ship, while cars, motorcycles, pedestrians and dogs weave their way in between. At times there seem to be half a dozen people shouting different directions to the drivers. But no trucks drove off the pier and all was loaded in quick time.
As well as the slower single hull ferries, there are fast catamarans and smaller, older hydrofoils. The catamarans are comfortable (many made in Western Australia and Tasmania), but do not offer as much outdoor viewing as the larger ships. The hydrofoils are like small aircraft with short wings to fly over the water. These offer poor viewing and a poorer ride.
One problem is access to the ferry terminal. At small towns this is just a concrete wharf you can walk up to, but in the cities, the terminal may be at the extreme end of an industry port area, with dimly lit potholed dusty roads. As the ferry may leave at 3am, it can be daunting to walk to the terminal (better to take a taxi).
Unlike an aircraft, you can board the ferry up until the moment it leaves. However, you can board ahead of time and it is more comfortable to wait on board, then on the dock, dodging trucks.
Some of the ferry companies are Minoan and Star. You book a ticket at a travel agent, much as with an airline or bus ticket.