Been using Generic Tool Store conveyor rollers for 4 years I believe. I dont ever try to dunk them in the river and they are protected from the elements by my raft for the most part. YMMV but I regularly push or pull a heavily framed and loaded 16' RMR (check the "heavy" box here) over them. use at least three of them and it makes it easy enough to pull the boat up straight.
+1 for the Ironton Roller and Bracket. I put three across the back of my trailer, and now can load and unload my boat MUCH easier. If the boat is unloaded except for the frame, it only takes one other person.
Game over, man, game over!