Loook at page 30 of the rulebook:
Submersible: … Whenever a round of combat starts and a submarine is in combat with only aircraft, it can submerge (before aircraft fire).
It says that it can submerge, doesn´t say it must submerge.
How can is possible such a mistake inside the first oficial errata?
Q. On page 29 it says that your aircraft may hit enemy submarines if you have a destroyer in the
sea zone, but it doesn’t say anywhere in the rules that aircraft can’t hit submarines without a
destroyer present. Do you always need a destroyer in a battle in order for your air units to hit
enemy subs?
A. Yes. Air units can only hit subs if there is a friendly destroyer on the battle board, otherwise hits made
by air units must be applied to units other than submarines. If you’re the attacker, destroyers in the same
sea zone belonging to your ally don’t count, since they’re not involved in the battle.
WRONG!!! The correct answer is: No. Air units can hit subs when there is not a friendly destroyer if those submarines don´t use the Submersible ability (the subs are not obliged to use Submersible. For example if they want to participate in the next round of battle with other naval and air units involved. Example 2: if there are only enemy air unists present subs can decide not to use Submersible and can be destroyed))