If you have a background picture of water then all you have to do is make it a sprite of a small size say 100 by 100 pixels in size.
After adding the small water sprite then clone it by moving the cursor over the image and holding down [Ctrl] then [Left] clicking and moving the mouse. A new instance of your sprite will appear. Keep doing this until you have twice as many clones as are needed to cover your layout. Example: If your layout is 1000 by 500 pixels you'll need 2000 by 1000 pixels of images. Then click the sprite and assign sine behavior to all sprites. This is done by selecting the image sprite in the Ojects window in the Contruct2 window. After asigning the sine behavior to the sprites then change the sine to a small number like Horizontal , Period 8 Magnitude 3. Keep tweeking until you have the look and direction you want, but it looks best when the Period is atleast twice the magnitude. Other sine behaviors can be added if a more chaotic looking water is desired.