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. Weed it and make sure the soil is decently watered.