Start counting from the next sea zone. AC to sea zone is zero. Island to sea zone is one.
Correct. I'd just add that a rule of thumb for movement in A&A games is that a unit only uses a movement point when it moves from one space to another. A fighter taking off from or landing on a carrier doesn't use a movement point for the action because it occurs within the sea zone. This is one reason why carriers are handy to have.
Unless it is going to put them at risk of getting picked off it is usually a good idea to get your planes in the air even if they won't be fighting to make use of all the possible movement.
Just a point of clarification: planes are always "in the air" during combat phases in Guadalcanal
, whether they move or not. I assume you mean that it's a good idea to move them off carriers or islands where there won't be any fighting for potential movement flexibility later in the turn.