Sup was used as a form of transport for the Polynesian people also useful as a fishing craft, however, SUP really got popular on the beaches of Waikiki, Hawaii around the 1950's where the "Beach Boy" instructors began stand up paddling on their boards with paddles.
It was then noticed by the beach boys that it was a very easy way to teach the tourists how to surf by making the student stand up on a larger board holding the paddle and pushing the students on to the waves. The sport of Stand Up Paddling was born into popularity after those few years back in 1950's.
Although the sport of SUP has always been around it never really took off until a few well known watermen such as Laird Hamilton & Dave Kalama started using SUP for as a great cross training tool as the fitness benefits are huge.
As SUP grew in Hawaii the sport's fitness benefits and health aspects started to get noticed, not to mention the load of fun SUP gives you.