You would figure a hotel of this size to have a shuttle service. Oh well, this is from Marriot's website:
Miami - MIA
Hotel Direction: 7.5 mi E
Driving Directions: Take 836 E. to 395 (Miami Beach) Exit. Exit to Biscayne Blvd, turn left. Turn right on NE 15th St. Go 1 block, turn left onto N. Bayshore. Hotel is on the right side of street on N. Bayshore Dr.
Estimated taxi fare: 23.0 USD (one way)
Depending on how many people you are travelling with, you have to take luggage capacity into account. You could call a shuttle service like Top Shuttle and it may be more convenient and save money as well. Here is a link to their website and you can get rates online: Top Shuttle