I can't stand leaving things half sunk. I'd put another fish into it to ensure that the weather doesn't claim the kill you earned. If it's worth one fish, it's usually worth two, says me.
In reality, maybe I'd let them go. But then again, it's war. Who knows? For your purposes, though, since nobody's going to die either way...send it to the bottom. The reason those ships exist, in game, is for you to blow them up. Use your tools so they can fulfill their long, plummeting destiny.
Besides, you might regret not having sunk it if you can't find any other targets this patrol.
Oh yeah...BE MORE AGGRESSIVE!
