Marmaris Bodrum and Kaş are Turkey's main diving centers. Bodrum is a great place to take a PADI course with good marine life and coral for experienced drivers. It also has plenty of wrecks and the possibility that a handful might open for exploration by the time you read this Marmaris has a larger more established scene,a legacy of its sponge-diving traditions. You can also dive off Kaş shores too and it's a much nicer place to base yourself.
A growing number of people enjoy diving off the Gallipoli (Gelibolu) Peninsula to look at old WWI wrecks.You can also dive in Ayvalik which is famous for its red coral.