Saltwater or murky water, intended as a nerf for “just dilute most potions”:

• Potions of saltwater behave exactly like potions of water, except that they cannot be used to bless or curse things. Holy (and unholy) water must be pure.
• All in-game sources of water except for magic fountains (possibly regular fountains as well, but they might be too plentiful) are saltwater. Diluting potions in them will yield only potions of saltwater.
• Can also get pure water if a unicorn horn dip would already result in water.
• Not specified what cancelling potions should do. Possibly most potions just cancel to saltwater, juice or sickness. It’s weird because flavor-wise it shouldn’t produce anything but pure water, but it obviously won’t fix the current problem if you merely have to cancel potions instead of diluting them.
• If called murky water instead of saltwater, the “murky” randomized potion appearance should change.

Alternatively, don’t actually make saltwater a separate potion; just use saltwater as a flavor reason for why the player can’t intentionally dilute potions to water. (Can use a separate flavor argument, that potions are generally corked while not used, to explain away the lack of dilution for water damage.) Also, make most potions cancel into non-water base potions (such as confusion and hallucination to booze), with the notable exception of polymorph. It’s also harder or impossible to get water from random alchemy or polymorph.

A third option is naming blessing-capable potions “purified water” and leaving the mundane potion of water as it is. Things such as unicorn horns can neutralize toxins but can’t actually remove them, so the resulting water isn’t purified.