It’s pretty common beach etiquette in my country.

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.