No, you don't need a car in the Marina; there's lots of public transportation (no streetcars or BART though) and tons of taxis everywhere. You can take the 30 Stockton or the 45 Union straight to Union Square. The Marina is also relatively flat so you can easily bike around, which is a great option for exploring beautiful Crissy Field and the Presidio right next door. There are lots of great shopping and dining options easily within walking distance on Chestnut, Union and Lombard, as well as a little further up Fillmore in Pacific Heights. If you want to have a car, you can park it around the Marina Green or find residential street parking, although it can be difficult, especially since a lot of the buildings in the Marina are apartments.
