Snuck into Hartlepool overnight, sank the lone destroyer, surfaced and used the deck gun on four ships in the harbour. By the time the inevitable response showed up, I'd sunk the ships, moved position and lay in wait rather than sneaking away.
Also, Fraps may be my only solution - I've got SHIII installed on a laptop (it's all I've got :P ) so the keyboard commands aren't working for me. Thanks again for the suggestions though, all.