Ah, you misunderstand! They already know how to "summon" water. BABBLER'S BROOK does that. This is a list of spells they already know about.
Some spells require extra items or ways to cast them (for example, summoning spells require an item and a cast phrase), but BABBLER'S BROOK is pretty straightforward, thankfully!

APPRENTICE: Read your book to learn SUMMON WATER.

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