Recently the Coast Guard made the following ruling:
“Based on the information available, the Coast Guard has determined that, when beyond the narrow limits of a swimming, surfing or bathing area, the device known as a “paddleboard” is a vessel under 46 U.S.C. 2101, and therefore subject to applicable regulations administered by the U.S. Coast Guard and its Office of Auxiliary and Boating Safety, unless specifically exempted.”
ie: Vessels are required to use a PFD.
I’m sure this will be a controversial issue as surfers not only prefer a minimalist view on gear, but boards are very buoyant, and if the paddler is attached to it by a leash, they’ll be fine. Most wetsuits these days also float, such as my 4/3 Xcel Infinity Drylock. Being a new sport in most inland water areas, it’ll take time for the CG and paddlers to resolve this issue. The Bend, Oregon based SUP blog Standuppaddle also has some info on this issue.
From years of kayaking and being in the situation of not being attached to a boat by a leash, there’s several reasons to have a PFD. If you get injured, or lose your boat, you’ll float, which helps if hyperthermia sets in or you’re too far from the shore to swim. You can can also use the PFD to carry flares, a VHF radio, a powerbar, light, a neoprene hood, etc in your pockets. Some come with a belt strap system to attach a tow or throw rope for rescues and towing. Up here in the Arctic cold waters of Washington, PFDs assist in keeping warm on and off shore. In busy urban areas where boating is popular, some come with reflector material that reflect sunlight or boating lights at night. I also attach a waterproof non blinking LED light on the back of my PFD for night paddling.