Well if we go by real life events, the British taskforce during the falklands war had an awefull time trying to acquire and sink a pesky argentine diesel eletric submarine (that if it weren't for defective torpedo or defective settings would have damaged or sunk one of the british carriers).
The British taskforce went on full ASW and dropped tens of torpedoes and never could get a fix on the sub. So I wouldn't really dismiss the lethality and survivability of a diesel sub against surface ships. And this was in the early eighties. Consider what a modern AIP diesel eletric sub can do, and you understand why the Virgina nuclear subs were developped.
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