The U-20 set was the original U-96 set used in Das Boot (1981). The Type U-19 of World War I and Type VIIC of World War II had similar internal dimensions.
The physical appearance in this production of Kenneth Cranham is probably more influenced by the appearance of White Star Line Captain Edward John Smith (famous as the captain of the Titanic) than by the appearance of the real Lusitania captain William Turner, whom Cranham is portraying. Unlike Smith, who's white beard is famous from still photographs and from Bernard Hill's portrayal, surviving photographs of Turner invariably show him as clean-shaven.
The character of actress Dorothy Taylor (played by Karen Haake) is probably more a nod to Dorothy Gibson, a film actress who famously survived the Titanic disaster. There was a film actress about the Lusitania, French-born Rita Jolivet, who (like Gibson) was later persuaded to appear in a film depicting the sea disaster that she had survived.