Welp...someones going to lose their license.

As Seagull said, the winglet does affect the "vorticity distribution"---as the engineers call it--at the wingtip, but it does so not by merely acting as a "fence" to stop the rotation, but rather by acting as an airfoil set at a very specific angle of attack in the local flow field. It creates lift (just like a little wing--or a wing-let...clever, huh?) which is directed mostly inboard, but also has a slight forward component which has the net effect of reducing induced drag. So yes, affecting the wingtip circulation is inseparable from the design purpose, but strictly speaking is an effect rather than the goal.

As for ground effect, yes, it interferes with the circulation from the wing, as well. Water, for the purposes of our discussion, is inelastic, and ground effect shouldn't be different over water or land--although I bet seaplane pilots notice it less because floats are usually a good bit taller than wheels, and keep the wing further from the surface.
 
Dugie, I read the article you posted (quite informative), and I class myself as a semi-convinced skeptic. My aero professor was a die-hard skeptic, but it seems that there are indeed some tests to indicate that in some cases, winglets might accomplish more than a span increase of similar length. However, they *have* often been installed and mis-installed "because they're sexy," or "by the marketing departments" and they're not a panacea. Even in this article, the author scorns aspect ratio increases for their increase in structural weight, but is also forced to note that winglets, too, induce a significant additional bending moment into the wing, exactly as a span extension would. That bending moment has to be reacted somehow, and that "how" is with additional structure. One thing's certain: there's still no monolithically accepted "best" in wingtip design. Look at the differences between a G-V, a 767ER, a 737, 787, an A-320, etc. My $.02.
 
thanks to you my friend, I have learned something here today, way to contribute knowledge...I look forward to seeing you and flying with you.

(so in a nutshell, the lower pressure the winglet creates in front of the wing offsets some of the drag created by lift, and the "fence" is merely a side effect...do I have it right now?)
 
Ehh...I'd rather say, in a nutshell, that the lift from the winglet is directed partially forward, which helps offset the drag created by lift. :-)
 
With regards to the decrease in induced drag also comes from having more lift at the tip and therefore a lower AoA across the whole span from the added tip lift.
 
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