"This is your water."
Translation:C'est ton eau.
Think about this: what does "mon, ma, mes" tell you? That there are 3 forms in French possessive determiners.
Remembering that French nouns can be masculine or feminine you may guess that "mon" is for masculine nouns and "ma" for feminine nouns. "Mes" looks like a plural, so it is probably for plural nouns.
From there, you will understand the following rule: "mon, ma, mes" are possessive adjectives which agree with the noun they modify.
If something belongs to me, "mon chien" (masc) is my dog, "ma fille" (fem) is my daughter and "mes parents" (plural) are my parents.
Besides, if a feminine noun starts with a vowel sound, in order to avoid the vowel sound conflict with "ma", you have to change "ma" to "mon", as in "mon idée" (fem., my idea).
The same applies to "ton, ta, tes" (your) and to "son, sa, ses" (his/her/its).
For "our", it is easier: "notre" if the following noun is singular and "nos" if it is plural.
For the formal singular or plural "your": "votre" if the following noun is singular and "vos" if it is plural.
For "their": "leur" if the following noun is singular and "leurs" if it is plural.