Você está correto - não há regras reais sobre isso, então cabe ao Mestre, mas se você quiser alguma orientação adicional:

Eu sugiro que, se você acha que há um momento entre perceber o que está acontecendo e se tornar incapaz de respirar, permita que um teste de resistência (provavelmente Destreza) prenda a respiração e segure-a antes que a condição que impede a respiração esteja no lugar. Se eles conseguirem salvar, respiram e podem agir sob as regras de "prender a respiração"; se fracassam, são pegos e vão diretamente para "sufocar".

Portanto, alguém que cai de um barco pode precisar fazer apenas uma salvaguarda do DC 12 relativamente fácil, enquanto um manto escuro caindo na sua cabeça pode ser um pouco mais difícil (ou um salvamento oposto ao ataque do manto escuro, possivelmente). Ser teleportado pode ser ainda mais difícil, possivelmente usando o feitiço CD do lançador, ou pode ser rápido demais para se evitar - isso depende de seu julgamento.

Como um aparte, polimorfo is an unusual case where most of the time it's to the target's benefit não to hold their breath, because the spell breaks if the new form dies, and suffocating is one of the faster ways to have that happen regardless of HP damage.

Cabe ao DM

There are no rules about that, so [un]fortunately it is up to the DM to decide what happens. And that's a good thing! Allowing the DM to craft their story, environment, and events are what roleplaying is all about. You can set the stage for what happens and how dangerous it is. That freedom is a feature, not a bug :)

When deciding, I would recommend staying consistent so that your players understand how things work and will work in the future.

As other answers have indicated, it's up to the GM, but here's some guidance on how the GM can decide...

Note that for a real-life human, "falling into water" and "falling into unbreathable air" are very different situations, due to the mammalian diving reflex. A person falling into water (even at the end of an exhale) will reflexively hold their breath, saving at least some oxygen... whereas a person falling into, say, pure carbon dioxide will probably exhale and go straight to suffocating unless they consciously know what is happening and choose not to exhale.

For that reason, I would rule that any remotely human-like player automatically holds their breath when surprised by a liquid, but goes straight to suffocating when surprised by an unbreathable gas.

Sim

The rules use the word “can”, not “may” so this is a capability you always have, not one you choose to engage.

For suffocation, the amount of air in your lungs is (almost) irrelevant - what matters is the amount of oxygen in your blood. For a healthy person, like an adventurer, the blood is 99 to 100% saturated with oxygen at all times so there is a lot of oxygen available e when you can’t breathe stress hormones reduce blood flow to non-essential body parts reserving the bulk of this oxygen for the brain. It is notable in this context that free divers’ pre-dive breathing techniques are intended to remover as much air from the lungs as possible.

Therefore, for game purposes there is no need to make a distinction.

Parenthetically, a suffocation challenge in the game is unlikely to be part of combat - it takes a minimum of 7 rounds to reach unconsciousness and I can count on one hand the number of combats I’ve been in that last that long.

I would also like to note that there are circumstances where a person doesn’t know, consciously or unconsciously that they should hold their breath. Being immersed in a fluid or exposed to noxious gasses does. However, insufficient oxygen or many toxic gasses does not.

Our breathing is triggered by a buildup of CO₂, not a deficiency of oxygen; if we are successfully expelling the CO₂ we will not notice the lack of oxygen until we pass out unless we are trained to notice the cognitive signs like fighter pilots are.

Similarly, many toxic gases are odourless, for example CO and H₂S. H₂S or rotten egg gas can only be detected at low concentrations; at toxic levels (which are quite low) it is completely odourless. Of course, these should use the poison rules, not the suffocation rules.

A possible simple solution:

You're either breathing in or out (maybe a 50/50 chance of either) at the time of transfer.

: If out, you have a second or two to react to the new environment. The GM describes the environment, emphasising the air situation ("You're in a room full of thick black smoke") and either it's up to you to tell him you hold your breath, or talvez if the GM sees fit, you get some sort of Wis or Dex check to do it before your next inhalation. If you fail, proceed to "In".

In: You're not holding your breath (unless you're in a vacuum), you're coughing up whatever you've accidentally breathed in. If it's poisonous, you've probably been affected by the poison. Maybe another sort of Dex or Wis check to avoid taking outro breath (harder than you think after you've just been through a coughing fit - I'd leave that to the GM's opinion)