I can't say what the vest and wetsuits are made of. We've been there twice, both times in Sept. Once in 2006 and last year. We basically wore the vests, however, last year, after the biggest storm I have every seen hit DC (and we've been to WDW about 10 times in Sept.) and after everyone was restricted to be under cover, and after the rain left, they reopened some of the areas. My DD11 and I decided to head back to the Coral Reef. Since the water is pretty cold and it wasn't hot out, we decided on wearing the wetsuits. They were great-kept us much warmer and since it wasn't hot and sunny out we didn't get hot walking around.
The park did close early that day however, the storm came back!