The name Marmaris supposedly comes from the Turkish saying "Mimari as", meaning hang the arhcitect. When you see the high rise package holiday appartment buildings you know what is meant by this.
The old town and the bazaar are found east of Ulusal Egemenlik Bulvari. Neither is particularly interesting, but the Castle Museum is worth a look.
Marmaris is mainly useful as a starting point for excursions to the main sights of the coast. It's a pretty useful base too.
There's plenty (some say way too much) accommodation, restaurants and bars.
Marmaris is easily accessible as a day trip destination from the Island of Rhodes, fast ferries operate daily, and the journey time is about an hour.