If you come to Kas, you need to see Kekova also known as The Sunken City. Overlooking the Sunken City is the  castle at Kalekoy (Simena). This is one of the nicest castles to walk to.
The gulet boat leaves from Kas Harbour at 10:30 and returns to Kas at 18:00
There is no music on the boat so you can enjoy the peaceful and quiet environment and just listen to the waves. 
Lunch is included in the price and is served on the boat.