There are restaurants near the hotel and the island does have some taxi's and trolley service. The hotel is about an hour drive from RSW airport and probably 30minutes from Naples airport. So Marco is a little difficult to get to, but the island does have things to do within walking distance of the hotel.
First let me commend you on your choice of the Marco Island Marriott. My wife and I really enjoyed the hotel and its wonderful beach! If you only want to veg at the beach and not get out much, then return the car and hang out at the restaurants and bars on the hotel property. Remember that one-way rental car charges can be severe; I suggest getting rental car quotes in both scenarios (returning the car vs keeping the car) and making an informed decision. You may be surprised by the results. If you keep the car, may I recommend a trip to Naples, FL? You will enjoy the shopping and restaurants on 3rd Street South and 5th Avenue South! Check out Campiello's Ristorante and Tommy Bahama's!
Hi Dmatas, First, the Marco Island Marriott is a great hotel- beautiful location on the beach, excellent service, and definitely a Marriott "resort". We have been there twice now. I'm torn about your car question- you don't need a car, if, as Money Advisor suggested you're willing to take a taxi once or twice. There isn't a lot to "walk" to, in my opinion. Verdi's is one of the best restaurants on the island but it's too far to walk from the hotel. Also there is the Esplanade area which has a nice bar/restaurant complex on the bay (CJ's & Tara Steakhouse) but you definitely can't walk there. The Marriott has Quinn's on the Beach for night entertainment and other restaurants as well. Hope this helps.
You don't necessarily need a car on Marco. The beach is out your front door and many restaurants within walking distance or a taxi ride. Check out CJ's on the Bay for a great outside gazebo bar and restaurant.