The captain has to announce the hit, so that the first mate knows that he has to check a damage box. Same goes for the engineer when he causes system or reactor damage.
When we played and this happened I did announce it to the other team, but my team mates got upset with me.
Our house rule is that all damage to a sub needs to be announced, regardless of the cause.
Just noticed that Arnaud from Matagot gave my reply a thumbs up (and 1 GG); I guess that means our interpretation matches his intentions.