Kekova Island

Kekova Island is located just off the coast near the town of Demre, Turkey. It's about halfway along the famous Lycian Way hiking trail that runs from Fethiye to Antalya. The Kekova region includes the island of Kekova, the villages of Kaleköy and Üçağiz and the ruins of several ancient Lycian villages. An earthquake in the 2nd century caused much of the area to sink into the nearby water. Today this is a popular spot for boat trips that will take you around the island to see the sunken remains. Some buildings are completely submerged while others are partially under water. There is also a Lycian necropolis that spreads out along the coastline, revealing half-sunken tombs. 
Kayaking tours and glass-bottomed boats are a good way to explore the area and get an up-close look at the underwater ruins. The island is a specially protected area, so swimming is not allowed. However, most boat trips will stop a bit farther from the sunken ruins so you can swim or snorkel outside of the protected zone. 

Practical Info

Boat trips can be arranged from Demre or Üçağiz, as well as part of a longer tour from farther away. Demre is approximately 90 miles from both Antalya and Fethiye and can be reached by bus from either city. The closest airports are Antalya and Dalaman.