I don't know if they're qualified as vessels (yet) by COLREGS. Stand up paddleboards are only vessels in non swimming-surf areas. If you're in a channel with a SUP, you're on a vessel and subject to COLREGS. If you're surfing or putzing around at the beach, you're on a surf board (no PFD
Kite surfers have quite restricted maneuverability and are subject to speed increases and directional changes that are beyond their control, even more so if they're new to the sport.
But in general I believe that yes, they are a person enjoying the water
, and have as much rights to be there as anyone else.
Hopefully COLREGS (no doubt lead by the USCG's rule
making engine) will clarify things a bit in the next couple of years.