Getting back and forth from Piraeus, the port of Athens, Paros or Santorini to Naxos is easy because the ferry company Blue Star Ferries operate all year round this exact route (and its reverse): Piraeus-Paros-Naxos-Santorini. Thus, the same boats that leave Piraeus twice daily at 7:25 am (morning boat) and 5:00 pm (evening boat), arrive first at Paros by 12:00 noon and 9:30 pm, respectively, then at Naxos by 12:50 pm or 10:30 pm (~5.5 hours travel time), and then, 2 hours later, arrive at Santorini. The morning boat returns immediately from Santorini to Piraeus at 3 pm, stopping at Naxos at 5:50 pm, at 6:40 at Paros, and arrives at 11:30 pm at Piraeus.  The evening boat stays the night at Santorini, departing there at 7:30 am, arriving to Naxos at 9:30 am, 10-15 am at Paros and at 2:45 at Piraeus. From Mykonos or Crete, connections are at least once a day and you will have to check in September for extra boats. Prices vary from 42 (any seat in the economy salons, like in the photo below) to 72 (businnes) euros and cabins with 2 beds cost 83 euros; preassigned “aiplane seats” are also available for 56.50 euro, but not highly recommended, as they do not have tables or couches, but are literally oversized airplane seats (see photos below).