Just wondering if an amphibious assault on a province where enemy have a supremacy. Do marine still able to land on beach or enemy naval bombers will pound the invading convoys to bottom of the sea?
For each strategic area you pass through with the invasion plan you must have sufficient Naval Supremacy. You get that by having sufficient amount of ships on patrol and by controlling the skies (air superiority). If you do not control these things you won't be able to invade (no sending your whole army to suicide). This makes it pretty clear now to defend as well. For example as UK - as long as your fleet is strong and stationed around the island and your air force is up and about you can feel safe from naval invasions.
Make sure to always have ships set up as escorts. Even if you have sufficient naval supremacy to launch the invasion - if the enemy rallies their whole fleet after the invasion is already under way there is still a chance some of the convoys could get intercepted.