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?