its not all plastic, but insulators are the things that foster static charge build-up. if you leave or pull electrons from the plastic you will be charged. then touching something that allows the charge to neutralize (like pool water) you'll likely get a small quick zap, not much different then dragging your feet on carpet (w/ or w/o shoes).
a wet Trex deck should yield little/no static charge build-up. a dry deck (a dry plastic insulator) will be different.
well, maybe, depends on how the conductive material is used. next is how to convince the folks who make Trex to make a new special product for "pool decking use, anti-static and bondable for safety".