The Tavern

If confused monsters can drown by stumbling into water, how come electric eels/lava snakes can't suffocate by stumbling onto land?

Lungs filling up with water is a lot faster way to die than gills drying out?

For lava snakes though... if we're positing a creature that lives IN LAVA, I can't see how being deprived of oxygen would be life-threatening for it.

And also things can kind of move around on the ground even if they're built for water. You fall into water that's too deep and don't know how to swim? You're going to sink like a rock.

I'm skeptical of the claim that monsters who live near water can't swim, but your argument that they could sink is convincing. I propose the following overhaul of the water system:

If a monster/player is wearing armour with ER>7 and falls into deep water, they sink immediately and die (flavour: their armour was too heavy, and they drown/are trapped until they die of starvation if unbreathing). Reason: This incentivises lighter armour and makes dex more desirable, which is currently often ignored.

However, if a monster/player is wielding a magical staff, spear, or trident, they won't sink unless their ER>10 (flavour: staves, spears, and tridents are made of wood and thus buoyant.) If the monster/player is wielding a two-handed staff/polearm, they can't sink at all (flavour: the weapons are long enough to reach the bottom and allow the player to hang scarecrow-like from them (https://bygosh.com/kids-classics/the-wonderful-wizard-of-oz/chapter-8-the-deadly-poppy-field/)). Reason: This incentivises polearms and staves, which need buffing, and adds complexity to the system to allow for emergent gameplay or something.

If a monster/player falls into water and does not sink, they are paralyzed for 2-4 turns and then moved back to the safe ground that they came from (flavour: they struggle back to shore (or fall back to shore if they had an appropriate 2-hander)). If there is only deep water surrounding them, they sink and die (flavour: they give up from exhaustion). Reason: this prevents too much cheesing of dangerous enemies (who aren't wearing heavy armour), yet allows early characters to drown the rare imps with GDA or similar, at the cost of losing access to said armour, providing interesting strategic decisions.

Any armour of ER>7 (ER>10 with appropriate weapon/no limit with appropriate 2-hander) cannot be retrieved from deep water even by aquatic races (flavour: it's too heavy to bring back up). Reason: this introduces non-trivial strategic inventory decisions: is it worth a slot to have the appropriate weapon to swap to just in case?

Swamp worms will be given the Confusion spell instead of Harpoon Shot (flavour: players are surprised at the sudden behavioural change in swamp worms that made them stop hiding underwater, and confused as to why it happened, evolutionarily speaking). Reason: Otherwise these awesome new water interaction mechanics will never really be used.

If a monster or player falls into deep water, any confusion status is removed (flavour: the water shocks them out of it). Reason: Drowning players via confusion is OP. And so is drowning monsters.

If aquatic monsters end up on land, they stop being confused and are paralyzed for 1-2 turns analogously to land-based creatures, and then can move back into the water (flavour: they need air and flop around uselessly for a turn or two, although it's easier to get back into water then out of it). Reason: The symmetry of the situation is aesthetically pleasing.

If a lava snake ends up on land, they freeze to death immediately and leave a (technically physically incongruous) ice block (flavour: they need lava's heat to live). Reason: Ice blocks provide meaningful interactive terrain—why limit their use to ice magic specialists?

If a player monster falls into lava, they die immediately. If they fly over lava or step adjacent to lava without rF+++, they die immediately. (flavour: it's lava) Reason: It's lava. Also, newbies currently don't value getting full resistances enough; this should incentivise players to try to get them more often.

Other features that could be included: relating the ER limit to monster/player size and strength, or allowing player to drink potions and then use the empty bottles as flotation devices, allowing them effectively to sacrifice potions to not drown.

In conclusion, I think that adding this to the game could shift the focus of DCSS from the same-old rut of endlessly slaughtering monsters that the game has fallen into, and open up more realistic simulation of the rich world of the dungeon.

FR: more fluids

Quicksand, acid, slime, differentiate fresh and salt water, tar, jello, etc

It's much easier to move from land onto water than it is for a creature in the water with no limbs to climb onto land.

Yes, I think that climbing vs falling is the main deal here. Falling is easier than climbing.