Marmaris is a cove and the beach is not particularly wide. It was originally all black sand but many hotel and other accommodation along with the bars and nightclubs have brought their own sand in so that they can attract guests to their sunbed and umbrellas. Their use is free if you are buying food and drink from then. Alternatively there will be sunbeds and umbrellas for hire.
This beach is just to the west of Marmaris and once the temperature starts to fall you might like to stroll between the two resorts, stopping along the way for a drink.
This beach on Sedir Island is almost unique in this part of the Mediterranean; it has beautiful golden sand but even this was seemingly brought from elsewhere by Mark Anthony for his Egyptian lover to enjoy.
It takes some effort to get to this sandbar on the road to Bozburun but if you want some time away from the crowds it will be worth it.
Paradise (Cennet in Turkish) Island is only really accessible from the sea even though it is actually connected to the mainland. Its beach is lovely and not particularly busy because of its relative inaccessibility.
Blue cruise Marmaris routes are great way to explore the islands.