Anyway, I borrowed a 5.5 hp 16 gallon shopvac. I was amazed, no shocked
at how fast it sucked up 16 gallons. Sucked out nearly 40 gallons to clean
out a 1200 gollon pond. Our pond is conc. I can't imagine using this with a
liner.

I use my shop-vac on my liners....

Me too. A liner held down by 100s of gallons of water isn't going anywhere.
~ jan