"de" is a preposition in its own right.
"du" is the contraction of preposition "de" and definite article "le" - to be used with masculine nouns: du vin, for example.
"eau" is feminine, so the partitive form is "de la", elided in "de l' " because "eau" starts with a vowel.
When followed by the definite articles le and les, de contracts with them into a single word:
de + le = du
de + les = des
But de does not contract with la or l'
de + la = de la
de la femme
de + l' = de l'
In addition, de does not contract with le and les when they are direct objects.
you may also look at this page, which covers the "partitive"case: http://french.about.com/od/grammar/a/articles_4.htm
If the word is feminine, such as (la) limonade, (la) confiture, then the French for some is de la :
If the word is plural (whether masculine or feminine), then the French for some is des:
Words beginning with a vowel
Before a word beginning with a vowel, use de l' instead of du or de la: