I was always wondering about underground rivers - their only purpose was to annoy the player that had no teleport or means of freezing the water (and a boon for gremlin bombers if found on D:8, but these guys are a dying breed now).
There could be another use for those.

Why not make potions that have been drunk leave bottles/vials afterwards?
Subsequently those could be refilled from any such river/lake and turned into useful potions of water.

One could argue that only bottles left after potions of water could be used in this fashion so as to prevent mixing new water from the river with remains of a previous concoction, but it would still be a progress.
You could also argue that in order to make it slightly more difficult to obtain large quantities of potions of water, you'd have to boil it first (who knows what vile goblins pissed into the river or how many orc corpses and rabid dogs took a dive), using perhaps a simple burning torch (or burning hands spell) that would require 3000-4000 action cost and always have a 10 - 20% chance of the bottle braking due to excessive heat/temperature difference?

As a lore-friendly addition, you could use the water from TotHK red lake that contains our favorite tiny, cruel-looking fish(es), and use it as projectiles to see those little beasts eating the face of some monster.
That would of course require some extra alchemical skills to avoid dipping your fingers into the lake or slipping and falling in like in case of pool-sipping, if you fail to make a concentration+dexterity check.
Also a higher chance that the bottle with cruel fish breaks in your inventory when a monster critically hits you or when you fall into a pit... this could give some flavor to the game and I doubt it would require any significant programming to implement since similar features already exist.

"The steel golem critically hits you. You are bleeding! Your cursed potion of cruel fish is shattered! Arghhh! The flesh of your groin is shredded by a swarm of the tiny beasts attracted by the smell of your blood! You die..."