Even though I live 40 miles outside of Miami, my wife and I stay at the SOBE Marriott at least once a year for a weekend away from it all. No stay is complete until you've had a meal at Joe's Stone Crab, which is on Washington street two blocks from the Marriott. Heading up towards the action on Ocean Drive, we also like A Fish Called Avalon, located in the Avalon Hotel. If you would like to have some Cuban food, try Lario's on the Beach, which is owned and operated by Gloria Estafan. Aside from these, just walk the gauntlet and you'll find something to your liking. The hotel itself is great and I'm sure you'll love it. The Conceirge lounge is excellent by the way and open on the weekends.
Next Time if you are staying at the Miami South Beach Marriott try dining at the Big Pink. It is great food at a reasonable price. Also, if you are a meat lover, try Fogo da chao down the street for lunch. It's only $26 and you will not have to dress up the dog only to chow down like one.