No one warned us before we went to Greece that many of the ferries--all the fast ferries that we took or saw--don't have any place where you can be outside while the ferry is underway. The windows aren't that clear--and on some you have an assigned seat that may be far from a window--so you really can't sightsee while traveling between islands on them. Fortunately, almost by chance, we took the Blue Star "Paros" from Paros to Naxos and then to Santorini--the most interesting part of our island trip--and on it we could be outside while underway.
I wish that I had known about this in the planning stages of the trip and made more of an effort to schedule ferries where we could be outside. (We have traveled on ferries in Washington State and Norway and never encountered this issue.)