There is a separate check-in line for Platinum Members, that you certainly want to use, but do check-in online first. Online check-in speeds up the entire process, but once you are at the port, the Diamond/Platinum line is much shorter then the regular line.
Miami is hard to predict when it comes to boarding. On numerous sailings I have gotten onboard shortly after 11:00, but I have also had to wait until 1:30 or 2:00 other times. There are a number of variables that effect boarding time. I'd give it a shot anyway.