I always put them on over a couple of coats of varnish and then add at least two more coats to protect the decal. Be sure you are using good spar varnish, never any poly varnish....
For the reproduction water slide decals like I buy from the WCHA, I put a couple of sealing coats on the deck, then the decal and more coats carefully on top. I only use Epifanes for finish bright work.