[Thrownatbirth]

it would prove very hard to find them without going active.

Not really - diesel boats create magnetic anomalies which can be detected. Minesweepers and various towed arrays can do the trick.

[stuartcr]

Yes, but they have to be relatively close, and if you are sitting under surface vessels, they can be fooled. Towed arrys are awkward in shallow, littoral waters, where there is a large fishing economy.