Central Marmaris is walkable — the seafront, marina, old town, bazaar, and Bar Street form one continuous strip a couple of kilometres long. Dolmuş (yellow shared minibuses) run constantly along the main road and out to İçmeler, Armutalan, and the surrounding suburbs; flag one down, tell the driver where you want to go, and pay a few lira when you get off.
For trips into the national park and the further bays, hire a scooter for around €15 a day (small rental shops on every block), take a water-taxi from the harbour, or hire a car. The mountain roads are narrow and winding but well-surfaced.
Taxis are widely available and metered, but agree the fare in advance for longer trips. Most hotels can arrange a private transfer or a driver for the day.