Hmm.. What are you doing in those shallow waters anyways? I think I would attack. It's night, the sky is clear, so the sea state might be favorable for a decent shot with a reliable estimation, even from a long distance. I would hope I got one lucky hit that would stop or even sink one merchant.
(However I have H.sie's and Stiebler's latest mod that makes one's boat harder to detect in shallow waters
)
It would be better not to get detected here. I would not attack with steam torpedoes during day light. If I couldn't attack at night time I would let the convoy pass.
Depending on the weather I think you might be able to sneak closer if you keep your bow towards the nearest escort, approach very slowly and deck awash.