i know the time has long passed, just for anybody else maybe reading this for the future.
my husband is working for yildiz yachting in fethiye and they are always pretty well booked, so i don't think there is a chance to just go there spontaneously and negotiate. too risky, also as the boats are very different from very small to very big ones. it would be a pity if you are only two people and have to rent a boat for eight or something like this.
would be interesting to hear if you made a different experience?