There are plenty of taxis at Miami International. I would not recommend the Super Shuttle as they have to fill the van with passengers first. They tend to stop at several terminals before you ever get underway. They only charge about $10 or $11 each, but if you have two or more in your party, you could have gotten a taxi (yellow cab type) for about $20 plus tips.
The taxi is faster. We got to the pier in about 15 or 20 minutes, this included the stop to show Custom booth official our I.D. (Sunday afternoon about 1:00pm). I think the distance is only 8 - 10 miles.
This was in July of 2002, so I don't have current rates or know of any procedural delays that may have occurred since then.