Water lake on landscape problem

Hi there,
I have this problem and I do not know how to fix it. I have a imported landscape and I cannot use the water system on it. By using the water-lake I found out that if use the lake on the landscape I only get the underwater post-processing and I do not have any water surface. But if I raise the lake quite a bit, the the water surface shows.
Have you ever encountered this? Is there a fix?
Thank you!