Terry Spragg and Michael Gardner get the prize for the best idea. I
actually considered this awhile back but balked at :

Salt water induced corrossion (solvable I admit)
Making semi-permanent mods to the dock
Oysters growing on the cable keeping it from working (prob not a real
problem)

Now, after your posts, maybe you mean for temporary use. Perhaps just
before a storm, at low tide, attach say 3/4" nylon to base of piling
and to upper end. Use a very thick UHMW plastic/glass reinforced
figure eight sliding on it (or something else like it) and attached to
the boat.

Only problem is that the boat can still rise only as high as the piling
top.