Hi,
I'd like to recommend a great day out that I had diving in Olu Deniz last week. I went on a dive yacht with an english owned company who had really friendly english speaking staff. The company is called Dive Med and they are the only diving company to dive from Olu Deniz. They have an office in OD - 3 doors down the main shopping street (with Hanel House on the corner) on the left (opposite Sultan Ahmet). They take absolute beginners and experienced divers. I'm sure you will also have a brilliant day out.