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.
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Cold with some icy stretches to start. There will be sunny intervals and further blustery, wintry showers.
Early wintry showers will clear to give a mostly dry day with some sunny spells.
Scattered wintry showers will continue overnight. Clear spells will allow a frost to develop by morning.