I'd expect you'll be okay. After all, there isn't a whole lot of difference between what you're going to do, and having the topsides of a boat get rained on for a week. I've used spar varnishes on wooden racing shells for years without problems, both above and below the waterline, and there's been plenty of times the boats have been wet for days on end.
Those are oil-based spar varnishes. Don't know if anyone's using water-based varnishes, but I'd personally steer clear of that. Also, no idea how it holds up to salt water. As someone stated earlier, the lifespan may not be that great, but you've got epoxy and three coats of varnish underneath. I'd recoat in a year just to see.
Turner Racing Shells Ltd.