Greg... its called FRP (Fiberglass Reinforced Plastic) and it does work fairly well.
Get creative with your floating material... barrels are great.... I used an old pedal boat one time... honestly, wood floats pretty good for a while... a few chunks of foam have floated just about everything we've ever built.
2 huge pieces of advice...
#1: Long... MAKE IT LONG... no a 16ft rail is not fun so don't ask LOL
#2: if you're doing something with an angle... make it mellow. The minute you think you've got it right... make it more mellow. Never fails, inexperienced builders always make stuff too steep.
Have fun... building/shredding rails is one of my absolute fav parts of the sport!