nah, no worries, mate! You are free to start being concerned when they depth charge you once you've finished your first patrol! There're enough fish in the sea and you should indeed keep in mind that hitting a merchant doesn't always mean sinking it. You either wait or shoot another torpedo once the rest of the convoy is out of sight.
When you are new I recommend event cam and free camera on in realism settings so that you can find out why you've missed. Having the torpedo crisis in mind it can be hard for beginners to learn and understand what went wrong!