We stayed at Marriott Biscayne for 2 nights prior to our cruise and we loved it. The hotel is dated, clean, and the staff are so welcoming and helpful. The airline lost my luggage and could not be delivered to the hotel before my departure on the cruise but the hotel kindy accepted to receive and store my bag until my return, although for the rest of my stay in Miami I had a reservation somewhere else.
Rooms are spacious enought, have comfortable beds and outstanding views (should you take a bay view room). The hotel is less than 10 min drive to south beach - $10 taxi ride + $1.5 toll should you take the Venetian drive. The same amount of time and money will take to get to Miami port as well.
If you like to walk (as we do) you can also walk to South Beach - we did, and it took us 1 hr at a slow pace.
I recommend this hotel to other travelers.