Miami beach marina is within walking distance of the beach. a little further south on key biscayne you can tie up at no name harbor, on the south side, and walk to Crandon Beach, maybe 1/2 mile. there are many trails, bike paths, etc... great week end location.
crandon beach is really nice, has often made the best 10 beaches list... MUCH nicer and more "tropical" than miami beach, FTL, etc...