It’s easy to get so involved with composing a shot, then setting all those camera controls that you would never think to use in “Auto” that you will not notice how the water line slopes from one side of the frame to the other. 
Slow yourself down to check the final scene before you push the button. Remember: When water is in the scene, even the slightest tilt will be noticeable.
Even slight tilts with water are noticeable.
Buy a cube level that will fit into the hot shoe on your camera, or use the virtual horizon setting if your camera has one built in. See — Think — Ac
There’s no benefit when everything goes down hill.