Teresa Cowley, who is from Mickleover in Derby was last seen heading towards the deck of the Pride of Rotterdam ferry in the early hours of Friday.
After visiting a bar, Mrs Cowley and her husband returned to their cabin but the door key did not work.
Mr Cowley went to a reception area to resolve the problem, leaving his wife sitting outside the cabin.
Police say CCTV and witness accounts showed Mrs Cowley went alone to a sun lounge on the ship shortly after 1am on Friday. Mr Cowley, who is now believed to have returned to the UK, is said to be too upset to speak to the media about his wife's disappearance.
Fears are growing for a woman who disappeared from a North Sea ferry during a crossing from Hull to Rotterdam.
Teresa Cowley, from Mickleover, Derby, was last seen heading towards the deck of the Pride of Rotterdam ferry in the early hours of Friday.
The 43-year-old's disappearance is being investigated by Harbour Police in the Dutch port of Rotterdam, who are liaising with Derbyshire police.
Mrs Cowley, who had no money with her when she went missing, boarded the ferry with her husband at about 8.30pm on Thursday.
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