Based on reports as told to the family by people on board a small resort dive boat from Dayang, there were a group of four OW students (including the victim, Winston Koh) and their instructor as well as a group of recreational divers with their guide. The students were doing their 4th OW dive and first dive Sunday morning. They were diving Rayner's Rock off Dayang. Rayners is known for its huge boulders, its sloping plunge from the corner of the island down to about 30m and, unfortunately, exposed conditions.
By local norms, the water was still a bit rough at the beginning of the season (there is a video on Facebook showing the divers entering the water). Estimates of visibility of only 7m or less is also poor by Rayner's standards but more like what is normal at the beginning of the season. I have heard from others at the site that morning that there was also a fairly decent current which, unfortunately, is fairly common for the site most times of the day / season. According to video, the instructor said (probably half jokingly) "hold on for your dear life" and "it will be better underwater". According to newspapers reports, the wife was told that the instructor told the students to all descend to 7 meters and he'd meet them there. When he got there (after an unspecified time), the victim was missing. The instructor looked briefly then signalled for everyone else to go up with him. They got back on the boat, searched briefly, took the students back to the resort a few minutes away. then resumed the search. The victim's BCD and mask were recovered that day but the body was not found for several more days. It was reported found at only about 100m away from the site.
Unfortunately, we will likely never know why the victim drowned, why his gear was off, why he got separated, etc. With quite a lot of experience teaching in the area though, I must say that the divesite was a poor choice for an open water course though. If sea conditions were ideal - which they were not that day and rarely are - I still would not teach an OW student there. There are too many other nearby more appropriate sites. Trying to keep track of four students at Rayners is just too risky, IMHO. Add surface chop, the small dive boat, low visibility, underwater current and some seemingly nervous looking students and it is even worse. That doesn't explain what happened to the victim but being at a better divesite would most likely have helped the instructor deal with the problem if it did happen elsewhere like Telok Jawa or in the channel just around the corner. Sad.