"It is his water."
Translation:C'est son eau.
A singular noun starting with a vowel or a mute H always uses the masculine form of the possession (mon, ton, son). This is to avoid the conflicting vowel sounds. Gender doesn't matter here.
sa eau→ son eau ma amie→ mon amie ta horloge→ ton horloge
But keep in mind that if an adjective (that starts with a consonant) goes between the possessive and the noun, the rules go back to normal.
mon petite amie→ ma petite amie ton grande horloge→ ta grande horloge