As someone who has had to read a LOT of pages of rules related to AAG40 (but WHAT a fun game!), the answer to your question is referenced here, in the AA 1940 Pacific Rules FAQ (page 4):
"Q. Letâ€™s say Iâ€™m doing an amphibious assault and there are no enemy ships in the sea zone
around the island Iâ€™m attacking, but the island has an airbase and enemy fighters. Can I move
extra ships and planes into the sea zone along with my transports that wonâ€™t support the
assault just in case my opponent decides to scramble the fighters to defend the sea zone?
What about if thereâ€™s only an enemy sub in the sea zone? Can I move a destroyer in along with
my transport just to keep the sub from getting a free shot at it, even though the destroyer will
not participate in the assault?
A. Yes, in both cases. You may move units into a sea that presents a danger to your units during
combat movement, even though they may not actually end up fighting. Just the chance that there will
be combat is enough to allow it."
Attached is that FAQ for your review, and happy gaming!