Thanks, I’ll have to read the instructions again. Now that I have played a little bit they may make more sense to me.
And read the Errata. As the OOB rules aren’t clear or correct (in many many subtle ways)
HOWEVER…. If your units are in a seazone that was neutral or friendly but is now hostile due to recently placed ships (or declaration of war, in your example), you can choose to leave that seazone as a combat move and ignore the enemy warships there (in effect, retreating before the fight). This is one of the very few, if not the only, combat move that doesn’t necessarily result in combat. So I believe that any units in the shared seazone are NOT blocked from moving from that seazone by your lone destroyer, even if he declares war. However, should he declare war, he HAS to move those units OR he HAS to fight the destroyer.
That only applies to shared seazones though. he would not be able to move ships from another seazone THROUGH that seazone with the destroyer as you cannot move THROUGH a hostile seazone. Only from it, or to it.