While filming on the RFA Robert Dundas (doubling for the SS Bernice), production there was interrupted when it was discovered an old brass ship's compass had been stolen. The culprit turned out to be none other than Jon Pertwee, who believed that the compass was to be scrapped along with the ship and so wouldn't be missed. In fact, it was to be auctioned off along with other ship artifacts.
Both Cheryl Hall and Jenny McCracken were earlier considered for the part of Jo Grant. According to the DVD commentary for the 'Special Edition' of the story, they were on the final shortlist of six actresses seen for the role. Barry Letts promised both actresses he'd use them in the future after casting Katy Manning.
Ian Marter (Lt. Andrews) had previously auditioned for Captain Yates. He would later play the Fourth Doctor's companion Harry Sullivan.
Tenniel Evans was a close friend and a long-time co-star of Jon Pertwee in the radio series The Navy Lark and had actually advised him to put his name forward for the role of the Doctor in 1969.