"Their cats drink water."
Translation:Sus gatos beben agua.
The way I heard it that kind of construction is not wrong but no one uses it so it will sound odd. You need to keep two things in mind. One people naturally like to use shorter notion (sus gatos vs los gato de ellos). Second in the real world there is usually context. You will already likely know you're talking about an "ellos/they" in the conversation. So "sus" will be enough to represent them.