The " horizon line" in a painting is where the sea and sky meet, at the eye level of the viewer, so if you are standing looking at the sea, it is one place, if you sit down it will shift higher. Here is the wiki definition if you want to get more technical and a picture of the horizon line from space
The horizon line is the first thing I put down in a seascape. I have lots more to say about horzions, but placement is the basis of the painting so we'll start here. Where you draw the horizon line dictates the point of view of the entire work, where you will stand in your own mind as you enter and enjoy the painting. It has great psychological effect and sets the tone and mood. After all it is where you are placing your viewer, so think carefully about where you put it. Placed high you have room for a foreground and plenty of wave action, placed low the sky will start to dominate. One of the painting principals I use is; What ever is most important gets the most real estate. So if I'm interested in the wave, it will have a big chunk of square inches in the work. I will not have room for a big wave unless I place the horizon line high.
Here are some samples of seascapes, some contemporary most from 1850-1950. Notice where the horizon line is, and how that affects the emotional state as well as you look at these.
Here we are standing on the beach, perhaps sitting on a small sandbank, your eyes are level with the true horizon
William Trost Richards, Summer
We know exactly where we are here and not worried about storms or big waves, riptides, so we can completely relax into this tranquillity.
It is possible to paint a seascape without any horizon line at all, as in this David Curtis painting a view common if you stand overlooking the ocean from a high cliff, but here our eyes are directed down, not out as in the first Aspevig work.