I, too, have heard lots of stories and have had only one encounter of my own regarding Florida boat registration. However, these are not just sea-stories. The law regarding the mandatory registration of documented vessels was passed into law recently (within two years). The out-of-state reciprocity law has been around (and enforced) for many years. Please note the last two paragraphs.
The following is straight from the FWC web site.
All vessels operating with mechanical propulsion devices (such as gas or electric outboards) are required to be registered.
Vessels must be registered and numbered within thirty (30) days of purchase.
Registration numbers must be displayed on the forward half of the vessel on both sides above the waterline. The numbers must be bold block letters at least 3" high in a color contrasting to the hull.
The vessel registration decal must be renewed annually and is to be displayed within six (6) inches of, either before or after, the registration numbers on the port (left) side.
Documented vessels without a state registration in full force and effect, must also obtain a Florida registration and display the validation decal on the port side of the vessel when using Florida waters.
Florida recognizes valid registration certificates and numbers issued to visiting vessel owners by other states for a period of 90 days. An owner who intends to use his vessel in Florida longer than 90 days must register it with a county tax collector. However, he may retain the out-of-state registration number if he plans to return to his home state within a reasonable period of time [he must still register the boat in Florida, however].