Synthetic lumber of some kind has been used for dock walkways and flooring for quite awhile. It looks good and seems to last awhile before succumbing to UV deterioration and becoming brittle.
My biggest hangup with it is that it creaks, pops, and snaps loudly when it flexes as you walk on it.
Light colored (gray or tan) SeaDeck foam is feather-light, keeps water from ever contacting the plywood, drys completely in 5 minutes, and it stays cool when sitting out in the sun. It's way better than carpet, but only time will tell how long it lasts.