There are half a dozen buoys that people have tried various methods. Most have chain, which goes very rusty and gets tangled up with the chains of the other buoys when they are put away. I've just finished untying the many knots in one chain. The chain is also difficult to remove from the propellors of the power boats it gets caught. Chain is more likely to damage the propellors. Why use chain? Why not use concrete or a brick or a big chunk of steel?
I need to allow for the tide to go up and down by a couple of metres, without lifting the lower weight off the bottom and without having rope floating near the surface. I think what will do it is a weight on the bottom of the pole, a weight on the end of the rope resting on the bottom, the rope attached to the bottom of the pole and a small weight part way down the rope to keep it tight and away from the surface.