Hey Al - Thanks for the suggestion.
I agree with posts above... in that... suction strength may [will??] eventually decrease. Product instructions on the link mention that any surface used must be wet... i.e."Item will NOT work on dry walls".
We too have a solidly fastened 1" line for transom boarding assistance from water. It's knotted every 12 inches for really firm hand-grip capability.
Also, I often use a 3" suction cup 'window glass lifter" all around both sides of our Tolly while sitting in water on a boogie board with fins on. This is done for cleaning boat sides with a extension handle wand and hand held kitchen scrub sponges for the dirty boot strap waterline. The easily adjusted position [to anywhere on the hull] large suction cup is used to hold on with one hand while I float alongside boat and work cleaning apparatus with the other hand.
Interestingly... The "cleaning wand scrubber-end" I use is an adjustable length handle spider-web/dust broom. It's light weight yet good for wanding water onto boat side; scrubs well too. Dirt falls off the spider-web dust broom with just a swish in the water. Works great to clean the higher portions of hull; while holding firmly alongside boat by one hand grabbing the suction cup's handles. This type of suction cup glass handling tool is shown at bottom of the link you supplied.
All of what I say - is - because 6 months of the year we have very comfortable temperature water to be in.