The word is su(water). It is the definite direct object of the sentence so accusative case marker will be affixed. Vowel harmony comes in at this point. There are four forms of accusative marker: -i, -ı, -u, -ü. The last vowel of the word is 'u' so it gets a '-u'. Regarding -y, that is a buffer letter. Some more information about vowel harmony:
There are two kinds of markers in Turkish which vowel harmony applies: The one with -a, -e and ones with -i, -ı, -u, -ü.
If marker is an -a, -e one,<pre>
the words whose last vowels are one of -a, -ı, -o, -u take '-a'. (adamlar, sorunlar, bakışlar, kollar) the words whose last vowels are one of -e, -i, -ö, -ü take '-e'. (ördekler, kiviler, banliyöler, ürünler)</pre>
If marker is an -ı, -i, -u, -ü one,<pre>
the words whose last vowels are -a or -ı take '-ı'. (elmayı, kalemi) the words whose last vowels are -e or -i take '-i'. (çileği, kirpiyi) the words whose last vowels are -o or -u take '-u'. (sonu, burunu) the word whose last vowels are -ö or -ü take '-ü'. (gözü, üzümü)</pre>
rawan, lucaturilli answers to your question in his comment just above yours. Should read all the comments because you will always find the answer. I do so since the beginning and i never ask any more question. You can go on Duolingo Turkish Grammar Portal (not yet complete). There you will find Selcen's explanations with a table with all the pronouns: https://forum.duolingo.com/comment/7736569
Anna.Lein, "su"="water" in "su içerim"="i drink water", "suyu"="the water" in "suyu içerim"="i drink the (definite) water". We have "limoni suyu"= "lemon juice" in "limoni suyu içerim"= "i drink lemon juice" and "limoni suyunu"="the lemon juice" in "limoni suyunu içerim"= " i drink the (definite) lemon juice"