> most sources that say this seem to draw it from the HBO series as if it were a documentary
That would be strange, because the series didn't show them die. And one of the epilogue screens said: "It has been widely reported that the three divers who drained the bubbler tanks died as a result of their heroic actions. In fact, all three survived after hospitalization. Two are still alive today."
[1] https://www.youtube.com/watch?v=nqOo3Rp7rCE