It is the salvation of death as is normal (since he already has 0 HP and should be obliged to save death).
However, this becomes a bit murky because he finds himself in a situation where he receives a round or more (according to the MOD WITH) before he starts drowning.
Initially, he is only doing salvage matches against the initial damage that took him to 0, so he would say he does the saves normally, but that each round he makes also counts for those NOC rounds before he chokes.
Unless you have a really high NOC score, it probably does not matter in the long term.
Situation 1 – WITH high (+3 or higher)
A high CON character may be able to stabilize the initial damage before drowning by achieving 3 successes before your NOC bonus in the rounds passes, but in that situation once the rounds of asphyxiation pass, death will begin again. (as if they were). start the saves again, since they stabilized the first set of saves (so 3 more faults are needed before death). This will continue until they reach 3 failures (since there is no stabilization while suffocating) no matter how many successes they have.
Situation 2 – WITH average or low (up to +2 WITH mod), or character WITH high that does not stabilize before the rounds of CON pass.
A character with an average NOC score would burn the round or two they would get by drowning before stabilizing, which would only mean that they would continue with their saves until they had three failures independently (again, since there is no stabilization while suffocating) without any Possibility of stabilizing even the initial damage.
A chain of 6 or more successes would only postpone death waiting for the end of three failures. That string of successes would possibly give an allied time to get to them and get them out of the water.