hello alena

you can see the 3 days wheathercast of marmaris by here:
http://www.meteor.gov.tr/2003/iller/marmaris.htm

and probably, it'll able to swim there in that month

i guess it's in 10 coldest and 25 warmest in a day as avarage in that time of marmaris

also it depends on in which day of october you will be there

and another stuff, alanya or antalya would be better choice in that season