Marmaris nightlife is concentrated in one place: Bar Street, the pedestrian strip a block back from the seafront. Three dozen bars and clubs run flat out from midnight to dawn in summer. It is loud, cheap, mostly British and Russian, and entirely focused on getting drunk; it is also one of the best-known party strips on the Turkish coast and has its fans.
For something quieter, the Netsel Marina boardwalk has wine bars, the kind of restaurant where you can hear yourself think, and waterside tables with marina views. The seafront promenade between marina and castle is good for an evening walk and a beer at a normal volume.