I don't know if anyone has raised this point before ... the Stats always remain the same at Litres of Water Used: 0
No, you are the first to notice this.

Thinking about this and the leader board, I think it is a good thing it works the way it does - If someone wants to give themself a bad case of carpal tunnel watering the plants manually, and if one of the ways a person can get on the leaderboard is by having a very high number for liters of water used, they deserve a spot on the leaderboard.

There is nothing stopping a player (with or without TCG running at the same time) from using the watering can to increase the-liters-of-water-used statistic. In fact, one could put the watering can over an empty pot (or even one with a plant in it if TCG is there to remove the excess water) and balance a weight on the mouse button to get the water flowing from the watering can, and then let it run overnight. Good thing it is only virtual water, otherwise you would wake up to a flooded basement.

It does seems wrong to waste water though, even virtual water.