Dont know about Marmaris, but we rent a house in Dalyan, i contacted all the Emlaks (estate agents) in the area, just keep emailing them every few week, they get to know your name and are very helpful.
Google long lets, try to do a deal with owner, if out of season will have more luck getting discount.
If you manage to sort it, when you get there make friends with the locals as everyone knows someone willing to rent for the following year, call into Emlaks, have cup of tea, chat, make your self known.
We were very lucky last year got 3 bed basic house/shared pool £200 a month, Sept till Nov, this year 3 bed house shared pool between 3 houses £400 month for 6 months.