To give something to drink to your cromimi, you need a drinking bottle (that you can buy at the pet shop, on the "cage" tab and then "drinking bowl") and water (that you can find at the pet shop, in the "food" section and then, "water")
Then, place your drinking bottle: in the cage, go to the "cage" tab and then click "drinking bottle". Click the picture of your bottle and place it where you want in the cage. Don't forget to save!
To fill your bottle with water, consult the console to the right of your cage.
Using the buttons in this console you can fill the bottles in one cage or all your cages. The prices for each action are listed on the buttons.