I realize that the ship transportation between the greek islands are like the bus transportation of countries with a huge continental territory, like Argentina for example.
Then, for the greek people of the islands the ships are more or less like huge (sometimes not so much) buses. So, the same for us the tourists.
Transportation to Paros by ship departs from Piraeus port every days with more than four or five frequencies in high season.
You can go straight to the port of Piraeus early in the morning (pay attention, firsts ships departs early in the morning, 7:30 at least) and you can buy the tickets right on the offices of the pier, or right in the small table in front of each ship or at all on board.
The price? 24 euros per pax ticket.
The line to Paros is the same for Naxos and Santorini. The trip lasts 4 hours, and several companies brings the service, like Ventouris, or Arkadia, Blue Star etc.
Data below about Blue Star Ferries, perhaps the most modern fleet.
We were stumped when we found out that there were no direct Ferry tickets available to return to Athens according to our schedule.
This was Aug 2010 and it was high seaso n in the Cyclades.
Our return flight from Athens to Dubai was already booked, and so we had to return that scheduled day. But there were no tickets.
In such a case, try out some alternate break journey schedule.
The options are through Mykonos or Tinos.
We got our tickets with a two hour break at Tinos. The journey time extends by an hour and a half too. But it is better to go through that, rather than missing an international flight.
This is an alternate option information which might come very handy when one is stuck.