Aliphatic for me every time, I use the standard type for most general use, veneers, deck planking, super structures and general joinery etc. It grabs quite fast and is sandable. There is also a wicking variety for those hard to reach places. I have also used Cyno for just about everything, if your not sure if it's suitable for the application you have in mind, try it out on a bit of scrap first.
Varnish as a protection for wood is fine for super structures and decking etc but I have never used it on hulls with exception of a final coat (matt or Satin) on a painted GRP hull.
Like Mad Mike, I would think that while several coats of varnish would protect the wood from water the hull would be very susceptible to damage especially if it was made from Balsa wood, many forum members skin their hulls with glass fibre cloth for strength. I have never done this so I can't comment further, I'm sure they would 'put you straight' if you asked the question.
Paul