It is likely that it is just a habitual thing, but I wonder if you've thought of mentioning it to your vet? Drinking excessive amounts of water can be a sign of kidney problems, especially if he's also peeing a lot.
If he doesn't pee much, then more likely it's a habit/fixation for him. I'd go with the suggestion of getting more water content into his food.
If he doesn't pee much, then more likely it's a habit/fixation for him. I'd go with the suggestion of getting more water content into his food.