If your dead set on a floating dock I know some guys put a walkway on their docks that are on a big hinge. The walkway can then float up or down as far as you need it on the hinges. Depending how far up or down you will travel will be how long your walkway needs to be. If that makes sense.
RC
That's what I did - and it works, for the most part. I do still get the dock in a bind after a real gulley washer - like right now for example - but it works well 95% of the time.