I can tell you they don't fock around with towing. I have seen duallys on the hook being dragged off. Your simplest bet might be going to the Marriot where the inwater powerboat show is, and using the valet, but they fill up quickly.
They will have shuttle buses running between all 4 or 5 locations (depending on if you want to count Strickly Sail as part of the boat show or not ).
As you drive across the causeway into South Beach, there are usually shuttled-parking lots marked by signs as well. They always have one near the helicopter tours place, right before you cross the causeway.
You can drive down to the convention center, and try to get lucky, but I would not recommend it. I normally rent a Harley for ease of getting around, but I will probably just use the yellow limo this year.