While in Oia we got a catamaran tour with Santorini Sailing and I am so happy I did. They have three options for sea tours and we picked the Dream Catcher main reason being the low cost at 95 euro per person. You will have the option of choosing the sunrise or the sunset tour. We picked the sunset, and I think it's the best, most romantic choice. The catamaran fits up to 30 guests and on our tour were 25. They will take the tour even if the maximum number of guests is reached. It is very clean and the crew very professional and fun.
The tour consists of sea travel, stops at the Red Beach, White Beach and the Hot Springs for swimming and a fabulous, all you can eat, home cooked (well, boat cooked) dinner plus the view of the sunset from the sea. On top of that you get free water, Santorini wine and beer the whole eight hour trip.