I seem to have read somewhere that the ship's helicopter had in fact seen a periscope, so presumably it was available. The TV programme is obviously heavily edited, and I am sure many events are omitted, or shown out of sequence, for reasons of security, brevity or good storytelling. Nonetheless, a towed array, however useful, does seem rather vulnerable. But probably that is a price you pay for effectiveness. And it makes for a very good TV programme...