PVC pipe is not UV adjusted and will eventually become dry, brittle, they'll crack and fall off. If all you are using them for is as 'guides' to mark the rear corners of the trialer underwater you might consider using the fiberglass antenna type 'markers' used by ATVers. They look like CB antennas and are about 4 or 5 feet long, usually with flags - the better ones have lights - on top (easily wired in to the existing trailer extremity markers). They also come with metal mounting brackets already attached. A couple of screws to attach them to the trailer is all it takes.
