Translation:The weather is dry. You need to drink more water.
Since 多 is an adverb, you cannot combine with a noun i.e. 多水 to literally indicate the amount. Otherwise, you are considering 多 as an adjective, which contradicts its purpose as an adverb. To correct this, include an appropriate adjective after 多 i.e.
Thus, 你要多喝水 is grammatically correct.
Think of it here as 'more' "You need to drink MORE water." But the MORE modifies the verb drink, not water. It is an adverb.