Raiding harbors did happen, so I don't see it as entirely unrealistic.
The USS Barb (SS-220) under Capt. (later Admiral) Fluckey once did it, escaping in broad daylight. Here's how she did it:
Quote:
Operating in areas suspected of being mined, the submarine maneuvered slowly along the shore and, instead of finding one or two anchored ships, discovered an entire convoy in the lower reaches of Namkwan Harbor. In the early hours of 23 January, she commenced an attack. Fluckey reasoned that, because Barb was so close inshore, she had to create instant confusion if she were to succeed in covering the ensuing hour long seaward run she would need to reach deep water. Hence, from a range of 6,000 yards, the submarine fired 10 torpedoes at the group of 30 ships. The skipper observed eight hits that sank three ships and seriously damaged three others. The smoke from the burning ships rapidly obscured the convoy and prevented further assessment of the damage inflicted. It also provided cover for Barb's escape. She ran at 21 knots through uncharted, mined, and rock-obstructed waters until, an hour and 19 minutes after she fired the first torpedo, the submarine crossed the 20 fathom curve into deep water. To make the best time, she remained on the surface and headed for Midway, where she arrived on 10 February.
http://www.history.navy.mil/danfs/b2/barb-i.htm
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Note at the bottom of the kill flag:
a train! That job was done by Navy Frogmen as the train was probably out of torpedo range.