I made a single rung rope ladder extension to my boarding ladder this year, to get it deeper in the water. Forgot the simple fact that wood, even plywood, floats. Remarkably difficult to get your foot onto a floating rung. I have now modified it by the simple expedient of fixing a length of old galvanised chain under the rung. I tested it in a water butt and it seems to sink satisfactorily, but I can only test it for certain when it is really warm next year, and I can face getting into the water. I have seized the ropes to the ladder as I couldn't figure out how to splice braided rope around a tube.