PC12 Captain experienced a NMAC east of MIA during a VFR ferry flight up the east coast of Florida.

Narrative

Decided to fly VFR up the east coast of Florida from south of Miami. We had planned on going up the coast; below the floor of the Miami Class B. Apparently everyone else had the same idea that day. We were just about the only aircraft receiving advisories and they came almost constantly for traffic all around. At one point Miami TRACON advised we had traffic at our 12 o'clock and less than a mile. We also got a TCAS alert and started a right descending turn. The traffic passed off the left wing but about 100 FT above and 300 FT laterally. I saw no evasive action from the other aircraft so I doubt they even saw us! We continued north bound off the coast to avoid most other traffic. When we were handed off to FLL Tower; they told us to descend to 500 FT AGL to avoid the departure corridor. We tried to stay at a higher altitude but it was too late to change course at this point. We dropped to 500 FT and were probably quite the sight to the boaters below because of our size and speed! We continued safely to destination but it was definitely a challenging flight. We complied with all airspace and ATC restrictions but there was little room for error. Next time I think I'll just avoid the airspace or go IFR!