It is at your own risk. I had everything packed in my backpack and when we were using the beach chairs and went into the pools I locked the zippers together with the little travellocks and secured it to the leg of the beach chair with another lock. Then everyone could go in the pools but we still kept an eye on everything.
At the beach we always tried to have someone sitting with our things. A few times everyone was in the water at the same time, so we just tried to cover everything up with towels and we swam right in front of our blanket and kept our fingers crossed. luckily we had no problems.