It is not a fast boat, it is an old ferry, but it will get you there...
The boat trip to Santorini starts around 9:30 from the harbour in Rethymnon to the island of Santorini and returns to Rethymnon around 21:30 - 22.00 in the evening.
The trip with the boat takes about 4 hours each way and there is a stop-over at Santorini for about 5 hours. You can join a tour of the island when there at an extra coast or you can just hop on the local bus ( there is a bus every 30 minutes ) and go to Akroti village.
The nicest village to see on Santorini is Ohi but it is too far from where the boat drops you ( and you have to change the bus in Fira ). Akroti is on the south of the island and is much smaller but also nice ( and very close to the boat ) and you can see Ohi and the rest of the island on horizon.
Wherever you go and whatever you do, DO NOT MISS YOUR BOAT!!!!

