As I recall, DAN has noted that in a very high percentage of dive fatalities, the diver died alone. That does not mean, however, that it was the diver's intent to be alone. In many cases, the diver was with a buddy or a group and somehow became separated. Of the people I know personally who died on dives, every single one died alone, but every single one started the dive with someone and had no intention of being alone.
I used to write the descriptions of cave diving deaths for the National Speleological Society's cave accident reports. Almost all of those were also alone. Most were planned solo dives; others had unplanned separations.