What is the use of the water tower in the mobile version of the game, other than using it in the nursery? Is it even worth upgrading?

There is also a recipe in the fish cafe that uses water. You either need to buy gold and buy plans with gold to upgrade it or just buy plans, both require real cash to have any impact. Worth depends on how much you spend on the game. You can easily play the game without ever upgrading it. Super feed from the nursery is nice, but not needed.