Theo Bailey, believed to be 31, joined his long-term girlfriend of 10 years Chantal Xerri, 27, for a night out with other travellers on Koh Tao last month.
Local police have often explained the deaths as accidents or suicides, but many cases appear suspicious to family members of the deceased, and are often left insufficiently investigated.
A young tourist (23) has vanished from the Thai paradise dubbed Death Island her.ie - get the latest breaking news, showbiz & celebrity photos, sport news & rumours, viral videos and top stories from her.ie Daily Mail and Mail on Sunday newspapers.