Which side: Black Sea or Mediterranean? Given the choice, go north. Either way, you're not going to find much in the way of secluded - at least not by my definition. But, there is a little marina about 2-3 hours east of Istanbul (by car) in Eregli. I don't know if you can charter there. But, if you can, that would be a good place to depart from. If you head east from there, after passing the lighthouse, it is a good mix of quiet, undeveloped coastline, beaches, small villages, and a few larger resort towns. So, you can kind of end your day anyway you like.
You didn't mention how long you planned to go, but you could easily spend a couple of weeks along that 100-mile or so stretch between there and Sinop, if you like to stay a day or two at nice locations. East of Sinop becomes less quiet.