Learn Hardcore Greek: Χρειάζομαι νερό για τη δουλειά.

In everyday Modern Greek the final -ν of the feminine article την is usually dropped before most consonants. Since δουλειά starts with δ, you get τη δουλειά. Keeping the -ν (i.e., την δουλειά) is not wrong—some speakers and formal styles keep it always. Rule of thumb: keep -ν before a vowel or κ, π, τ, ξ, ψ and the clusters μπ, ντ, γκ, τσ, τζ; otherwise it’s often dropped.

Why is there no article before νερό?

Because νερό is a mass noun here and you’re talking about water in general. Greek often omits the article with indefinite mass nouns. If you say το νερό, you mean specific water (“the water”). You can add a quantity word, e.g., λίγο/αρκετό/πολύ νερό (“some/enough/a lot of water”).

Can I say “για δουλειά” without the article?

Yes.

για δουλειά = “for work” in a general, non-specific sense.
για τη δουλειά = “for the job/work” referring to a specific job or the work at hand (often “my work” by context).
You can also make it explicit: για τη δουλειά μου (“for my work/job”).

Could I use στη δουλειά instead of για τη δουλειά?

They mean different things:

για τη δουλειά = purpose (“for the work”).
στη(ν) δουλειά = location (“at work”).
So Χρειάζομαι νερό στη δουλειά means “I need water at work (there),” not “for the purpose of work.”