This is a common problem I've found in a few jobs related with ocean. When the camera is close to the surface, looking at the horizon, those black areas appear. Either with a normal texture or not.
It looks there's a problem with ocean's self reflection, but I can't avoid this by adjusting the bounces. Any ideas?
Here is the .blend file. As you can see, the environment is visible only through camera and glossy, and the problem keeps with a blank environment too. The scene is lit with a sun lamp and plain color environment.