True--as with anything on
Amazon, there is a lot of "no name" stuff that is pretty bad quality. I've also bought my kids no-brand water shoes on amazon that broke the first day they were used, but doesn't mean there aren't ones that work. I bought the "Aquapac" one and it has held up through dozens of trips--pools, water slides, snorkeling in the ocean in rough waves, you name it. I'm sure there are others that are good as well.
For anyone nervous, you can test it by putting a tissue inside, sealing it up and putting it underwater. Stretch it, move it around, leave it for a few hours, then wipe it off and open it to see if the tissue is still dry--that should tell you if it is working or not.