Local men Chris Murray and Gordon Maclean will take the plunge to lay a wreath on the wreck of the Iolaire, which was dashed on rocks during a storm in the early hours of 1 January 1919 while carrying soldiers and sailors home to the Western Isles after fighting in the First World War.
The special ceremony will take place directly after an official memorial service attended by Prince Charles and First Minister Nicola Sturgeon, as well as descendants of those killed in the tragedy.
The loss of the men, many from the same families in Lewis, Harris and Berneray, was a further blow to communities already decimated by deaths in the combat.
Hundreds of people are expected to turn up for the event, including Scotland’s most senior naval officer, Rear Admiral John Weale, and Norman A Macdonald, convener of Western Isles Council, being held at the Iolaire Memorial in Stornoway.
Organisers said the service would make a “right and fitting” tribute to those who died.
Professor Norman Drummond, chairman of the Scottish Commemorations Panel, said: “It is beyond our comprehension that over 200 men perished so close to home after surviving the war in what remains one of the worst UK maritime disasters of the 20th century.
“When you look out from the Iolaire Memorial to where HMY Iolaire hit the rocks of the Beasts of Holm, you are struck by just how close they were to shore.
“It is hard to imagine the relief and excitement of the men and their families on their return, and then the sorrow that was to follow.
“It is right and fitting that we hold a WW100 Scotland Commemoration in their memory and reflect on the lasting impact this tragic incident had on future generations on the Western Isles and far beyond.”
At the end of the service, to be conducted by Very Rev Dr Angus Morrison, the prince will unveil a new sculpture to commemorate the tragedy – a bronze depiction of a coiled heaving line that references the heroism of John Finlay Macleod, who swam ashore with a rope to rescue 40 of the 79 men who were saved.
It was created by artists Will Maclean, Marian Leven and Arthur Watson, and will bear the names of those who were lost and the communities they came from as well as a bronze wreath composed of maritime insignia.