get up the lake bank
Are your parents in wheelchairs? You may be better off building a paved ramp. If this is private property then you will need to conform to ADA Standards, which you will be able to check out at www.ada.gov. Depending on what state you are in you may have to involve an Architect. If this is public property though I would look into the town making it ADA accessible.
With a ramp you need to keep in mind that it will have to have railings on boths sides and 5 foot square landings and maximum slopes. This always eats up a lot of distance, like 200' or more.
Either way it will be expensive. I'd speak with an architect if you are seriously considering this project. Good Luck.