Obvious-in-retrospect addition to the research plan: it's possible that the crystals are what's causing the water to have its properties, so we ought to study the crystals too. I recommend taking samples, waving our chakrascopes at it, and adding "rune that recreates this crystal" to our list of prep days.
Rune That Recreates This Crystal
Manipulates nearby stone to bear all the physical properties of the sample crystals harvested from the Otter Cave. The type of stone the rune is designed to convert is either red granite or extant runecrafting-grade substrate, whichever Hazou thinks has a better chance of working.
Of course, it could be possible that the crystal is literally just substrate, chakra-conducive in a way that the cave entity uses to Do Things, and thus transmuting substrate into this crystal is a meaningless operation, but if so that's fine.