I've flown off a beach a few times, I confess I didn't ask anybody for permission, but I did pick a smelly and deserted beach in the middle of nowhere.
Two things I would mention. One is that it's the best opportunity you'll ever get to judge how good your own landing was - as you taxi back you can see how hard, how straight, etc. Second is that my experience is that wet sand with small ripples tends often to reduce apparent ground effect, so be prepared for an exaggerated flare to avoid a hard landing.
Great fun.
G