The aforementioned cases seem to refer to conscious drowning rather than shallow-water blackout.
In a well planned CTB by drowning, time to unconsciousness can be managed by auxiliary means. In particular, breathing concentrated N₂O (or other asphyxiant gas like nitrogen, butane, or tetrafluoroethane) may cause fainting within half a minute. Asphyxiation with simple asphyxiants reduces blood oxygen saturation similarly to drowning, but much faster and typically without unpleasant symptoms. So basically it's possible to use 2-stage asphyxiation to CTB:
1) gas asphyxiation to induce loss of consciousness painlessly,
2) asphyxiation by water (after consciousness is lost) to cause death.
N₂O has an advantage over N₂ because of additional dissociative and analgesic properties, mitigating any possible disturbance in the semi-conscious state.