Looking for skilled wood worker?
Answers:
No nothing will work forever. You can seal the surface of the wood, even treat beneath the surface with oils. Eventually they all need reapplication. Polyurethane is very popular, they make two part epoxy primers that last a long time. I prefer natural solutions, like boiled linseed oil, or tung oil.
Other answers:
Spar marine varnish is very good. I use it for outside doors.
Spar marine varnish is very good. I use it for outside doors.
It depends on the application.
Using a naturally resistant wood like Cedar or Tamarac won't need treatment beyond what you'll do for cosmetic purposes (stain, etc).
If you need a decorative clear finish, Spar varnish is what's used on boats.
For something that isn't decorative, cupric arsenide is what's used for pressure treated wood, which is used for sunken posts without rotting.
What are you trying to accomplish? Wooden boat makers apply fiberglass and resin coated with marine grade poly. This will make it weather and waterproof. Most everything else will make it water resistant at best. Hard to tell you the best option without knowing what you are trying to do.