The original plants fishing game
To have water available for your seeds, you need to have a Water Tower. The game has various models with different capacities. You can get a free Water Barrel to start in the HQ. It can hold 10 units of water and comes with 5 units.
The water in Alienum is obtained following the real water chemistry formula: H2O.
If you are using the Greenhouse HUD, you will see two numbers: oxygen and hydrogen. Hydrogen you earn during extracts, since it is the most common element in universe. Oxygen you need to buy, but don't worry, it is cheap and you can earn it during extracts sometimes.
To produce the water is simple: have your tank rezed, touch it to open menu, and select 'Produce'. A new menu will open with the quantities of water you can produce, and a 'Custom' field to enter any amount (remember that the tank will not produce more than it can hold). The tank will need 15 minutes to produce one unit of water using one oxygen and two hydrogens.
You can have more than one tank to hold water. The quantity of water informed by the HUDs are the sum of water available in all water tanks you have. And you don't need to have a tank rezed near the seed plots, you only need to have water on it.
Note for users from versions before the 4.0: if you had water (the one earned during extracts) in your account, it had a part converted into oxygen and hydrogen, and the rest in water credits. Every time you ask the tower to produce water, it will get the water from your water credits and just put into the tank to fill it. It will repeat until your credits ran out, then you need to start making water in the normal way. (You can check your water credits in the Greenhouse HUD).