What possible reason could there be for leaving food you intend to eat unattended in public? Do you do that often? Where do you live that you can trust it wouldn't be taken or otherwise fucked with in that time?
I left my wallet and phone next to the pizza (in a bag, they weren’t visible), if people were going to take the pizza it’s likely they’d take the actual valuable stuff first.
May also help to know the beach I was at was in the wealthiest sub in my city. Petty crime is pretty uncommon at any beach though. People leave valuables while they go swimming all the time.