I have an uncle who has a beach house that had a 3nd floor deck. He put a deck on the 2nd floor below it and didn't want the water dripping down. I believe he waterproofed the upper deck by covering it with exterior plywood (?) and then had that fiberglassed. The fiberglass was then protected with an expoxy paint. This was about 7-8 years ago and has stood up to the harsh beach environment without any problems. Much lighter than concrete too.
