Leash. And why do you need a leash? For example for the dog? So that the dog does not run away. And the same can be transferred to snownoards, surfboards and SUP boards. You attach your stand UP paddle board with a leash on your foot. So the board can't go off, if you fall into the water.
And here are some interesting facts about the topic Stand Up Paddling:
The pleasant cool refreshing breeze, which blows into your face and over the body during the SUP is an argument for the meditative effect of Stand Up paddling. The absolute reflection and concentration on your own body, the nature around you, the immersion in another world, let you completely relax during the paddling. You can also continue with yoga on the SUP board. But be careful: you can get wet!!!