A diesel boat could also shadow another surface ship closely enough to completely blank out it's sound signature. A surface vessels sound signature is all fuzzy and distorted. A submerged sub in deeper water is "clean" for lack of a better term. They just light up the diesels and snorkel along at 60 ft or so... At night they can run full ahead and they look just like any other pig boat! (surface ship = pig boat)
It's not easy but it's plausible. Again, as Tom mentions, fuel is the problem.