Re: polyester or nylon anchor rope?
For 20' deep water you're probably going to need at least 50' of rope or more, I prefer a 3/8" or 1/2" Braided Nylon because the larger size and the Braided rope is a lot easier on your hands than a smaller diameter or Twisted type rope, and it doesn't seem to tangle and kink up as much. Depending on the type bottom and speed of water current would determine type of anchor and length of rope. A 3' or 4' section of chain between anchor and rope is also a good idea to make the anchor hold better and keep the rope from rubbing itself in two due to wave action, if you have any. Good Luck!