In the first sentence
I want some water to drink
"to drink" is an infinitive acting as an adjective modifying "water". It is telling us what kind of water you wish. You want drinkable water, not water suitable for cleaning floors for example.
In the second sentence
I want to drink some water
"to drink some water" is an infinitive acting as a noun phrase which is the object of the verb "want". "Some water" is the object in the infinitive noun phrase "to drink some water".
"it" is not required or grammatically correct as you have proposed.
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