Got my deck and hull together on my Petral Play SG.  Still long way to go.  But I do have a question about varnish.  I made a few surfboards back in the 70s and I finish with epoxy and polished it out.  I didnt use varnish.  So why is varnish recommended as final coats on Kayaks?

Is any marine varnish good...or recommendations of MFGs. 

Kayak is dark stained on deck and natural on bottom...no paint

Thanks in advance for any information.