It depends on which pass you are planning to buy:

• Interrail Greece Pass - unlimited travel within 4 days (or any number of days depending on the pass), you may use it on any train between 0:00 and 0:00, and you can choose on which days you want to use your travel days
• Interrail Greek Islands Pass - same as above, but only valid on ferries (of participating companies). You can take as many ferries on a single day if you want (if that's even possible). So you can take a day trip from Naxos to Koufonissi or Santorini for example. You could even use it as day trips to the Sporades or the Saronic Gulf Islands from the mainland. Note that for accomodation like a cabin you need to pay a surcharge, usually on overnight ferries. (overnight ferries are usually Piraeus to Crete, East Aegean and Dodecanese) \\ More info here: https://www.interrail.eu/content/dam/pdfs/greek-islands-pass-guide.pdf