I like this idea.
Sometimes, especially in the chaos of a 4v4, I just plain forget to tell my crew the general plan.
A gunner who can hear that the friendly Galleon is targeting the enemy Goldfish without me having to relay anything would be pretty nice.
However, all that chatter might be distracting for the average crewman, so maybe a pop-up in the lobby: "Your Captain has selected you to be his First Mate! This will allow you to hear the Captain's chat, but not contribute. Do you accept?"
That way, both parties need to be willing to participate.