Flying low over water creates a great effect in footage. I know the Phantom 3's sensors had some problems with flying over water, and they had to be disabled. Is this still true in the Phantom 4 with the collision avoidence system?

Yes, it's still true for the Phantom 4, the water can affect the normal function of the vision positioning system. The environment requirement should be the same as the Phantom 3.

When flying low above water, aircraft tend to tilt more. Be careful when you accelerate, keep a safe height.