I've had this problem in the past but worked around it. This time I can't. I have a plane object with water material on it. There seems to be a threshold where the renders no longer render correct the closer I put the camera to this object. And though the camera is not touching it in anyway it renders almost completely dark. When i raise it up a bit it renders fine. Unfortunately in order to pull this off I need the camera much lower.
Probably there is fresnel involved in the material which makes the material less transparent and/or more reflective as your angle to the surface gets farther from perpendicular. Reduce or eliminate the fresnel so you get the same look from all ( or bigger) angles