I tried these:
Home â€¢ PILE RING
They do everything the maker says... and they seem to be really well built, strong, etc.
But they didn't work especially well for us on the forward pile. The angle of the dangle -- from bow cleat to water level -- just wasn't right. Too steep, so it was difficult (impossible) to keep the sides of the boat, further astern, from banging into piles on the opposite side.
I also tried mounting above water level, to reduce that angle. (And if water level raised to float the boat higher, the boat would pull the ring higher up the pile.) Worked a little better at rest, but not better enough to continue using them. The "boat pulling the ring higher" wasn't all that great.
Some dock neighbors use Tide Minder balls, and they seem to work OK; the angle to where they hang on the pile is up to the user.
I suspect Tide Slides would be great... but they're pretty proud of their systems.