This example illustrates how to diagram an infinitive being used as an adjective.  In this case, the infinitive is modifying the noun, "water".  It is answering the question, what kind of water?...the water to drink.

It is diagrammed "like a prepositional phrase". (But remember, a prepositional phrase would have a noun or pronoun following it as its object - NOT A VERB - thus, you should be able to tell whether the phrase is an infinitive or a prepositional phrase.  "To" on the slanted line below the subject, "water" and the word "drink" on the horizontal line.  It is not on a standard because it is just an infinitive.  When you look at the other infinitive diagrams, you will see an infinitive phrase on a standard.

Water to drink was scarce.

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