Usage: WATER <garden keyword> <water source>
You can use water containers (i.e. waterskins, teapots, water jugs) to water your garden, but it is a waste of your time and effort.
Instead, acquire a water fountain from an in-game vendor.
Keep an eye on your garden, especially if your skill level isn't very high. You might not realize you've over-watered it until it's too late.
It's a good idea to log in once per real life day while your garden grows to make sure they are decently watered.
If the garden is too dry it will stop growing and the growing time in the GARDENS command will stop increasing.
Watering the garden will start the timer going again.
Weeds will grow more if the garden is dry or over-watered.