Like Nick said "standard" marine epoxy will definitely degrade over time due to UV exposure. There are several makers of epoxy that offer epoxy with UV filters (eg West, Resin Research, Super Sap, Raka). A lot (a majority?) of surfboards are finished with just these epoxies. They really extend the life of plain, uncoated epoxy.

I am sceptical of "UV" filtered resins for surfboards. I've seen specifications of 1000 hours of UV exposure. While this is plenty for a foam surfboard which is basically a disposable item, but it isn't that great for a kayak. Again, if you are going to be careful and protect the boat in other ways you can probably get away with it, but I would still want a UV protective finish over top.

To add some UV protection one option is to add some West 423 graphite powder to the epoxy mix. It's also supposed to add some "slipperiness." I've finished a couple of race hulls that way. You can have it in any color as long as you like black...

You may also need to be careful with graphite powder. Black is not the ideal color for a boat that will be out in direct sun. It will heat up something fierce, softening the epoxy underneath.

I am also highly doubtful of any "slipperiness" qualities of graphite in a finish like this with respects to skin friction against flowing water. For it to do anything it would require enough friction to actually slide graphite molecules off the surface of the boat. Slippery against rocks, maybe, against water? highly doubtful.

Again see Mike and my comments regarding making a boat slippery vs getting the paddler strong. You will have much better results spending time doing sprints than spending time polishing or trying to otherwise tweak your boat.