I bought these shoes for a vacation in Hawaii, and unfortunately, they did more harm than good.  The problem is that the heel portion that rubs against the Achilles tendon is too stiff.  So, when sand splashed into the shoe, a little sand stuck to the material and rubbed up against the back of my heel and achilles tendon and gave me painful blisters.  The only way I could have prevented that from happening would have been to wear calf-high socks, but that kind of defeats the purpose of wearing water shoes.

After that vacation I bought some water shoes from a Japanese company (not sold in the US as far as I know), which had a softer, more flexible heel, and I had no problem at all with blistering on my next vacation to the beach.  I threw the Speedos away.