Definitely - ALWAYS sit on your board, it will be easier to attempt a relaunch.
You wont get the foil in your lines.
And last but the most important - you can sit on your board and just drift ashore, without having to swim for your life if out far and not much flotation (say in shorts and a tiny waist harness only as we do in the summer).
You will also save your board that wont drift ashore and hammer on the bottom, if rocks it might get destroyed.
Even better, sitting there you can enjoy the others riding around you, and IF a puff comes, you can try and sometimes succeed a relaunch
A win win win - sit on your board no matter what, when kite is down