Unfortunately those are profoundly unsafe. They only keep the child's head above water while they are slid all the way up to the shoulders. If they start to slip down the arms, which they are apt to do when swimming, they'll tend to slip all the way down to the hands and if the child isn't strong enough to pull themselves up out of the water it can keep them from being able to swim at all as it holds their hands up.

Try to imagine if you were less buoyant like if you had ankle weights on and someone tied two empty milk jugs to your hands. Your hands are suddenly not useful at all for swimming and you can't pull them underwater so now you're forced to hold yourself up by pushing your arms out.

Here's how it can look, and this makes it a bit clearer why it can be a hazard. https://i0.wp.com/renomomsblog.com/wp-content/uploads/2015/0... If it slips down to the wrists like this it's arguably worse than having nothing at all.