Here is the link to the local bus schedule from the Port.
You can transfer to the C or S bus which runs to Miami beach every 10 minutes at Biscayne Blvd. Get off at Lincoln Road (it is a pedenstrian street closed off to traffic) and walk one block to the beach and art deco district. The cost is $1.50 and takes about 20-40 minutes. On the way back, be sure to ask your bus driver to announce the stop to get off at. There is a lot of re-routing around contruction on the way back and if you miss the stop you will get a tour of the more "interesting" areas of downtown Miami (although the bus does eventually loop back around to Biscayne BLVD)