It really isn’t rocket science, but it’s an absurdly common mistake. A mistake at the most basic level. The last time, and every time I’ve ever looked, the horizon is horizontal.
There’s a clue in the origins of the word HORIZON – HORIZONTAL
It never ceases to amaze me just how many images I see, even from people who claim to be competent photographers, where the horizon is enough out of kilter in a landscape or seascape to make me (metaphorically) foam at the mouth! It’s simply nothing more than a sloppy and careless mistake.
As you can see in this image (which has been tilted by only two degrees), the mistake is especially pronounced when the main subject of an image is the sea!
I can’t claim to have seen all the oceans of the world, but having stood at different points in my life admiring the Mediterranean, The North Sea, The Black Sea, and the Indian, Atlantic and Pacific Oceans I can confidently testify (aside from the curvature of the Earth which can be barely seen by the naked eye) the horizons of each are all, without exception, horizontal.
This is one of the simplest mistakes in the world to avoid. Take your time. Look. Concentrate. Use the auto-focus points or other viewfinder elements as reference. And, if in doubt, if you aren’t confident in your own ability to level your camera take several images adjusting the angle slightly to cover yourself.