There are no car rental outlets at the pier in Miami. And if you don't want to tour Miami Friday night, no need for a rental.
The cab fare from the airport is a set fare per cab (not per person). Think it's around $25. And a taxi from South Beach to the pier may be about $15 -$20.
Less hassel than dealing with rental returns.
Now if you want to use the rental Friday, there's a different story, and I can recommend some rental co. with offices not too far from the port.