The Greek Islands are amazing, but when you are traveling by ferry there are a few things you might want to keep in mind.
Depending on the time of the year, some ferries will not run to all the Islands multiple times per day.
The trips can be quite long so bring snacks.
Again depending on the season the seas can be quite rough and the ferries may anchor mid trip or not show up at all...
So my advise is plan on spending several days on each Island and plan on being very flexible.