Spar varnish can use either polyurathane or natural resins and oils, with urethanes being more durable. The difference between spar varnishes and others is that they're more flexible to resist cracking when used on...well...spars.

I've had no trouble with cracking with either Zar satin urethane (not spar varnish) or Minwax Helmsman (spar varnish).