I have a 4 acre pond which has been in existence for 35 years. It was a sand pit and when excavated they hit springs. It is lovely, full of koi, bass, turtles and WEEDS. I would like to make a small area very swimming friendly. Maybe something the size of 2 very large swimming pools. The deepest part of my pond is about 17 feet - if I can just find a nice way to get out there, I would be so happy. Can anyone help me by suggesting what I might line parts of my pond with?