UV radiation does more than discolor epoxy, it kills it.

Paint or varnish with UV protection is necessary.

You might want to look at The Epoxy Test and The Epoxy Panel after 1 year.

I built a pirogue that I never varnished and it lasted fine for the 10 years that I owned it, but there were some special conditions. First, I used System 3 epoxy which was one of the better performing ones in the test that George mentions. Second, the boat was stored inside a garage when not in use. Third, it was rarely used in the winter and usually only in the morning (between 7:00 amd and noon) in the summer. In the summer, it was almost always paddled under a leafy canopy. And I live & paddle just about on the 40th parallel. So it never really got major UV exposure. Finally, because of the scratching caused by hard use it was typically sanded and recoated every couple of years, so no part of the finished really lasted 10 years without varnish.

The boat did change color, yellowing a bit, but I did not find that objectionable.

I can't say what would happen with your brand of epoxy, nor might you want to be reapplying the outer coat every couple of years. Your best bet would be to varnish it, at least 3 coats. If you do the preparation properly, you'll be gratified at how nice the boat will look after the varnish is on.

Have fun,

Laszlo