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.
More top news
An engineer has been confirmed dead in the crash in Wales. The RAF say the pilot survived and is receiving treatment.
A Red Arrows jet has crashed after an incident at RAF Valley in north Wales, the Ministry of Defence has said.
It has been confirmed that the Red Arrows pilot survived the crash but unfortunately, the passenger, an RAF engineer, died.