Hi,
The water dragon in relation to apartments can be a funny thing. I too am in an apartment. Your feng shui is typically oriented to your personal living space. On page 70 of your level one manual you'll find directions on which way your water dragon should flow based on the degree of your front door. The best way to represent it would be with a bubbling fountain outside the door if possible or just inside the door. A picture representing the optimal flow works as well as Astrid mentioned.
How do you feel when you view the river from that particular deck? Marie does not address much of the landscape feng shui save for what is mentioned in level one on page 52. If the land behind the apartment slopes downward, this can be symbolic of abundance rolling down the hill away from you. It's remedied by placing some sort of sturdy support at the back of the property. It can be symbolized by rocks, trees, shrubs ultimately you are holding the consciousness of having support at your back that the universe is there behind you, it's got your back.

All the best!
Kat