I prefer full foots every time for snorkelling, and not just for warm water or sandy beaches. It's quite possible, if you get them in the right size, to wear protective footwear with them. And if there's any danger of them falling off, then fin grips can be purchased, straps that fit over the heel and instep when wearing full foot fins.
When I started diving in England the 1960s, full-foots were the fins of choice, while open-heels were widely regarded as being for those who couldn't afford full-foots. It's a little known fact that when Beuchat came up with jet fins during the 1960s, they produced them as full-foot fins. It was the American Scubapro company which decided to manufacture them in an open heel design, maybe in part due to the greater popularity of open heel fins in the USA compared to Europe. Full foot fins were invented in Europe, by Cressi in Italy.
Having given my reasons for preferring full foot fins, I wouldn't try and influence anybody else's choice of fins, as they are very personal bits of gear. I do believe, though, we ourselves are strongly influenced by the gear we used when we first learned to dive. Maybe that's why I enjoy snorkelling with vintage gear so much.