Seu is for masculine words and sua for feminine ones.
Água is feminine, so use sua.
- Seu(s) = your, his, its.
- Sua(s) = your, her, its.
So,sometimes we may confuse if the possessive is related to the 2nd person or 3rd person. Then, in 3rd person you can use dele (his/its) and dela (her/its) after the noun.
- Sua família é legal = your/his/her family is nice.
- A família dela é legal = her family is nice.
Now, it's easier to understand the owner. ;)
seu(s) and sua(s) are the 3rd person possessive pronouns.
Even though você(s) means "you" (2nd person), it is grammatically 3rd person for historical reasons. So the real question would be "How can
your if it means