Can dominate monster and drowning be used as an insta-kill?

Question

In a convention scenario that I played in, creatures used dominate monster on a player character, and taking "total and precise control of the target", ordered the character to jump into a convenient pool of water and inhale deeply. On the character's subsequent turn, the character jumped into the pool and inhaled deeply. The DM then said that the character dropped to 0 hp and died instantly with no death saving throws.

Can dominate monster work this way RAW?

It appears that dominate monster can be used to make the character do something obviously suicidal. The relevant part of dominate monster states:

You can use your action to take total and precise control of the target. Until the end of your next turn, the creature takes only the actions you choose, and doesn't do anything that you don't allow it to do.

This could be used to allow someone to jump off a 200' foot cliff (although, in D&D, that would not necessarily be fatal to a high-level character), jump into a pool of lava doing an arbitrarily high amount of damage, or stick one's head into a sphere of annihilation, or, as in this case, to drown oneself in a pool of water.

Suffocating takes a certain number of rounds of no oxygen to begin dying, and then requires death saving throws. 3 failures will kill you, but 3 successes won't stabilize you:

When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.

So the question might reduce to whether death by suffocation can be sped up (drop to 0 hp on the current turn, skip saving throws) by intentionally inhaling water. But if it doesn't, are there other interactions between dominate monster and intentional suffocation, such as gaining a new saving throw when suffocation drops the character to 0 hp, or no longer being dominated once unconscious but still suffocating?

Answer 1

Dominate monster probably does work this way.

The spell description seems pretty straightforward:

You can use your action to take total and precise control of the target.

This seems pretty unambiguous to me, the instructions given seem to fit squarely within "total and precise control".

Drowning definitely does not work this way.

The rules for suffocation are unambiguous. Let's review them in order, comparing them to the rather unfortunate ruling the DM made in your situation:

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).

The dominated character was ordered to exhale all of their breath and suck in water, so holding their breath does not apply here. However, the next rule is where your DM diverges from the rules:

When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round).

Your DM ruled that the creature died instantly. The rules say they can survive a number of rounds equal to their constitution modifier. The next rule takes us even further away from the DM's ruling:

At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.

At this point the creature starts making death saving throws, and does not die until they have failed three, per the usual rules for death saving throws.

So the DM's ruling is simply incorrect with respect to the written rules.

Answer 2

No, you can't drown someone instantly this way

D&D is not a physics or biology simulator. It is a game with rules, and the rules do what they say. The rules say what you quoted:

When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.

They don't say anything about speeding thing up by filling the creature's lungs, or anything like that. They are arbitrary and simple so you don't need to memorize different rules for drowning in water versus drowning in sand versus drowning in air (for a water breather). You just look at Con bonus, and don't need to worry about size or relative lung capacity, or other physical adaptations that might make a difference. The rules do what the rules say, and they do so to make it as simple, concise and enjoyable as possible for the the players.

Drowning doesn't work this way even in real life

Water in the lungs doesn't make you drown that much faster. Human lungs take a good portion of the oxygen the first pass, and can only take so much more by holding unmoving air. They can also take oxygen from water or other fluids, but the oxygen content in normal water is much lower than air. Look up oxygenated breathing fluids if you are interested.

The point of suffocation, is when the concentration of oxygen in your blood reaches a reaches a point where it is too low for your brain (and the concentration of CO2 is too high). That is when you start to pass out and your brain shuts down. Once your lungs have extracted all the oxygen they can get (and passed out all the CO2), that starts to set in. Having water in your lungs will only make that slightly quicker. Panicking and thrashing around, burning though energy, has a far greater effect then what is in your lungs at the time.

To that point, one can counter that domination would slow down suffocation, because the dominated creature is not fighting it, unless instructed, so they would run out of oxygen more slowly.

But again, D&D is not a biology engine, so that does not come into play, either.