South Pointe Park, at the extreme southern end of Washington Avenue in Miami Beach, has a few spots with views of the main channel into the Port of Miami. Both cargo and cruise ships will pass here. There are several large condo towers, but there are a few places where you can walk close to the water and have a good view. Take the MacArthur Causeway from Miami to Miami Beach, follow to Washington Avenue, turn right, and go to the end of the road.
Yahoo's or Map Quest's on line map services will give you a good idea of the area. type in Washington Avenue in Miami Beach, FL, then click to move the map southward until you see the ship channel.
This is a great location. Enjoy spotting!
Regards,
Charles