@ghetty Here’s a couple videos I made about MAP:
MAP is covered at 10:55:
https://www.youtube.com/watch?v=yFhtShzVL2A
MAP Interdiction of Submarines at 2:52:
https://www.youtube.com/watch?v=Q-DaBgO_XUk
Before France falls, if you put a British plane on MAP with some naval units, you can force any enemy sub entering that sea zone to attack you. That means the sub (as the attacker) will have to decide if s/he wants to roll once or submerge. Then all the British naval units get their defense rolls. Most German players won’t enter a MAP-patrolled sea zone that contains a small attack force like a heavy cruiser, a light cruiser, and a couple destroyers because they are likely going to get that u-boat sunk. With proper placement (say SZ’s 3 and 24), you can prevent u-boats from getting out after your convoy lines. If you don’t have a plane on MAP, the submarine just waltzes through any sea zone that just contains naval units.
After France falls, you have to worry about the Germans hitting you with their air force, so you may not want to continue your “cordon” strategy, and the “First Happy Time” begins for the u-boats. At this point, you might assign some escort ships in sea zones that are out of reach of German aircraft, but you are likely going to take some convoy raiding until Britain develops Advanced ASW. While awaiting that tech advance, MAP can then be used in a retaliatory manner as you note above (with MAP +/- Destroyer).
As others have noted, if your aircraft started the turn on MAP, it cannot move to another sea zone for a different MAP mission. It would return home during the non-combat movement phase.