I'm no liner pond guy, just a dreamer of schemes, but if I were wanting a Swimming Pond, I would consider making the hole as deep as I could, make the sides as steep as possible with smooth stones as the substrate that sits atop the liner. The perimeter would likely need to have some shallows to promote some emergent plants for filtering effects. The depth and the side slope would be determined by your pond liner guy's design capabilities. I would put a swimming platform or dock in and go swimming.
Deep water is not conducive to plant growth so long as the water is not crystal clear. The lack of dirt/sand in the pond reduces this as well, I would think.
A aeration system and/or fountain will help keep the floaty stuff to minimum and assist with any biological breakdown of debris by putting 02 deeper into the water.
There are also "natural" filtering systems that could be incorporated at a later date if need be.
Image search "natural pools" if you need some temptation to go nuts with it...there are some amazing examples out there!
