Hi Steve
I made the journey up the Dalyan river from Marmaris by boat a few years ago, very enjoyable it was too.
There are certainly Mullet in the river. I doubt ifthere are too many at Dalyan though, as there are heavy netting operations downstream. I'd still have a go though. You can almost certainly rent a small boat from Dalyan, to get away from the busy riverfront in the town.
The open sea is really a waste of time. There are small reef fish, small Mullet, Bream etc close in but nothing much over an ounce or2. There are one or 2 larger fish offshore, I once saw what I think were Tuna rounding up a school of bait fish. You'd be very lucky to catch one though, and there is no organised fishing.
Good luck, enjoy!