I have made a few. They tend to break. It takes awhile, but the material does fatigue and break. The worst case was a tail I took on a 2 week cruise to Hawaii. First day out, 5 minutes into my first swim in the ship's pool, the fin broke. (I bought some novelty pencils at the gift shop and borrowed some duct tape from the crew. It was enough for one swim, then I had to repair it again.)
I have moved to polypropylene. It will still eventually break, but it takes longer. Its cheaper, and the same material used by FinFun.
https://www.mcmaster.com/polypropyle...lypropylene-6/