Always book in resort. much cheaper.

Not been to Turkey for a while, but the boat trip we did to the mud baths in Dalyan was good.
Actually done it 3 times.

We usual go down to the river and negotiate to get the best deal. You can get a boat to yourself depending how many people there are but you must haggle. The advantage of the boat to yourself is you decide where to go, like the mud baths or turtle beach. The price will decide on the size of the boat. Never had a Turkish bath in Dalyan but would imagine some of the larger hotels would do them.