I want to do one of those silly puzzle traps, like have a character dropped in a pool of healing potion, or even waterbreathing. I'm hoping that after the panic subsides, then they still have to worry about getting out, with the long term threats being starvation.
Can a character drown in a healing potion?
Page 183 of the PHB describes what it takes to suffocate:
A creature can hold its breath for a number of minutes equal to 1+ its Constitution modifier (minimum of 30 seconds.)
When a creature runs out of breath, 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.
If you run out of breath, you can’t regain hit points or be stabilized until you can breathe again.
The bare minimum of breath holding is going to be 30 seconds, which is 5 rounds while in Combat - quite a bit of time if you are using Combat time to manage the puzzle trap.
You do drop to 0 HP, and are unconscious, so there really isn't a chance to drink the healing potion you are currently drowning in - and you can't gain healing points anyways while suffocating. Leading you to be surrounded by the very life saving material you need, but can't use.
The suffocation rules are applicable and clear here, however, you might have to check the errata to get them all. They are as follows:
A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).
When a creature runs out of breath, it can survive for a number of rounds equal to its Constitution modifier (minimum 1 round). At the start of its next turn, it drops to 0 hit points and is dying.
If you run out of breath, you can’t regain hit points or be stabilized until you can breathe again.
As stated, they can't regain hit points until they can breathe. They may drink the healing potion all they like, but it will not help them stabilize. It may take awhile for them to no longer be able to hold their breath, but once that occurs, they will die fairly quickly.
Regarding your potion of waterbreathing option, that will fall into DM discretion. If the DM believes that a potion is substantially water, then maybe they can breathe it. Though, I assume if a trap is made out of this potion, then the DM's view is that it isn't water and thus not suitable for respiration.
You could make it a potion of water breathing, as you already said, or something similar. Arguably most potions are primarily water (as are most drinks in the real world) with extra medicinal or magical ingredients. So in a pool full of water breathing I'd expect you can breathe.
In the real world, it is possible to breathe while submerged in a heavily oxygenated liquid. Presumably, you could have a magical potion do something similar, even if you have to customize a rule (ie. use the, "at the DM's discretion," clause).
