I would guess that it was ill first. My young blue phase White's didn't spend that much time in the water. They acted just like adults, hanging out on the glass or on a leaf all day.

I don't know if it was sick and that made it easier for him to drown, or if he was sick and just died, but my guess would be ill (Just a guess from what you described). I would guess it was before you got him. I'm so sorry about your loss.