Wax on a kayak has many downsides, for all the reasons Rob mentioned. However I have been known to wax the deck for a show, but otherwise steer clear of this practice. I have used automotive style waxes just fine. I doubt there is any added UV protection other than the minor increase in shininess. There may be some advantage to using something like 303 Protectant, but I have never used it on varnish. Also, if you ever apply any product containing silicone, you may permanently hinder the ability to refinish the boat. So be careful what you use.

: Do any of you wax your boats? Which wax?

: I guess it would help keep varnish looking fresh longer.

: Joy dish soap removes wax so thats no issue.