Let them approach closer in order to make sure no escorts or other surfaced threats are in the mix. If not, start a submerged attack and eventually surface to continue with torpedo attacks and perhaps throw in the deckgun.
Since i don't know what location you are at and if you are playing GWX or stock SHIII i can't advise you on any airborne threats. If you are playing GWX and are within range of airborne patrols i'd keep an eye out as they can pack a heavy punch.
It would seem however you can have your merry way with this convoy. Good hunting
