It has been about 8 years since I was in Marmaris, but there was a restaurant that was run by trainees for the food industry, I think it was called the school house or something , it was very good. Also , if you walk into the harbour area where all the posh boats are moored - it is kind of round the corner at the centre of the town, there were some lovely restaurants round there that did a bit more than the usual tourist fayre. A little more expensive, but I remember everything being so cheap that it didn't make a great deal of difference.
I would recommend the seafood and they also did some slow cooked casserole & rice dishes that were absolutely delicious. Wash it all down with Efes lager. Have a great holiday.