We're traveling from Santorini to Naxos and then on to Piraeus. What is the situation with the luggage? Do I need to reserve a cabin to store our luggage or will there be lockers? We are traveling with Sea Jets and Blue Star ferries. We will have large international carryon back packs ( ebags motherlode).
You have nothing to reserve for your luggage.
In conventional ferries (Blue Star Ferries) as well as in almost all Seajets you leave your luggage, and especially the biggest ones, in the vehicle deck on dedicated racks when entering the ferry
There are labels on the racks indicating the destination islands. Find the racks that matches your destination.
Don't worry, the ferry staff will help you find the right rack if you get lost.
Note that in high season the racks are sometimes not sized to contain all the luggage, if there is no more space you can put them on the ground in front of the racks
Once you have dropped off your luggage, you go up to the passenger decks.
Your luggage is safe, no one has access to it as soon as the ferry is on its way. Keep with you what you will need for the crossing as well as your valuables.
In your case the exception could be with the Santorini-Naxos crossing if you take the small Seajets catamaran "Superjet" which does not have a vehicle deck. In this case you can take your luggage in the cabin with you, there are reserved spaces to put your luggage.