It is true that Marmaris is very commercialised and caters mostly for the Brits, however it will be much quieter in October than in the height of summer. The area around the harbour is very nice, if you walk right to the end, it has a lovely feel about it with some nice shops.
Don't know how your boys are about walking, but the walk along the back of the beach in the opposite direction to Icmeler is very nice.
Perhaps walk one way then get a taxi-boat back ?
Someone mentioned you can get a dolmus to Turunc, a good day out would be taking the dolmus one way where you will see spectacular views as you go over the mountain, then the taxi boat the other way.
If you are adventurous there are many good trips you could do on public buses from the bus station in Marmaris - journeys that we have done are to Datca, which is a lovely very Turkish town at the end of the peninsula, and also to Mugla the capital town of the area. You can also get a bus to Akyaka which is a small resort (again very Turkish), with a great sandy beach, and shallow sea that your boys will love.
You could also get an organised boat trip to Dalyan, again a fantastic beach and beautiful scenery.
So if you find that Marmaris isn't really for you, there is loads you can do very easily - Turkey is one of the best places we have been for getting out and about, their transport system is great and very reliable.
Have to say that you may be disappointed with the wine, its definitely not the best in the world!
Nevertheless, I am sure you will have a great holiday,and you never know you may love the resort, lots of people do!