HopCar, you may be brilliant!!! I am at anchor
but I of course had some WD40 at hand. I tried it on a logo patch (nothing to lose) and it worked a treat! It worked so well I proceeded to do the rest of the sticky mess. Lots of rubbing and lots of terry cloth rags were needed because they gummed up with the sticky goo. But it seems to have done the trick. Now I'll give it a good coating with Aero 303 Protectant and keep an eye on it to see what happens to the glue joints.
By the way, this PVC dinghy
is only 2 years old but it's been in the Southern California
(not tropical) sun continuously with out any cover. As I said the tubes and joints were okay, only the glued on bits like rub strake, oar locks, logos were gooey.
I'll report back in a week or two. For now, wow! Thanks HopCar, and everyone else who offered helpful ideas.