with regard to price,from fetiye it was about £7 each under 5s were free.
find the boat yourself maybe wait till the boats are coming in on an evening and ask some people who are getting off.
your own instincts will tell you what suits you best,some boats are very big and lively and some are not.
personally we went on boats holding about 50 people although they were not full.
so it was comfy,the food was fine, fish or chicken with bread and salad.
we went on 3 trips and all were excellent value and a good day out.
although we went from fetiye i dont think you will find it any different from marmaris.
dont buy from the boat selling ice cream unless you want your first £5 ice cream.
dont book thru your tour operator you will pay them far to much and you dont get anything you wouldnt by doing it yourself.
some of the car hire/trips shops offer a good deal but like with everything in turkey confirm what you are getting then haggle a little.
maybe go check out the boat they are offering.