The beaches in Marmaris Downtown are not the greatest in the world but are more than adequate for children and adults alike. The sand is mostly black sand and a bit gritty.
The beaches are not very wide and the top half is covered in sunbeds and umbrellas which you can rent for a nominal fee if the hotel/restaurant does not provide them. Many hotels have private beaches and some even ship in very good sand from other areas. The sea is warm, crystal clear and perfect for swimming.
There are some fabulous beaches within half an hours drive, so if beaches are your thing, visit Cleopatra Island where the sand is said to be the best in the world. Icmeler Beach is just 15 minutes away wich is a wider beach and it is much quiter then Marmaris Beach.
Another World Faumos Beach is Dalyan (Caunos or Turtle Beach) it is about 10 Miles long Beach. You can read more here