We began our Greek trip by flying into Santorini and from there took ferries between the islands.
Useful information when travelling by ferry:
Book ferry seats online in advance either via DirectFerries.com or directly with the ferry company. There are different levels of ferries – the fastest are the most expensive.
You should be fit and able to board the ferries. You carry/wheel your own luggage onboard and it is a little chaotic at times with passengers and vehicles boarding by the same ramp. Good idea to keep luggage to a minimum.
Luggage is left on the racks provided on the lower decks of the big ferries and passengers walk to upper decks to be seated. Some ferries have lifts but many have just stairs.
Ferries are clean and comfortable. There are restrooms plus a kiosk on board to buy food and drinks.