For this topic, I have combined these subjects for a very good reason and that is, sometimes it is necessary to include some part of the sea, or coastline in a landscape, just as you will sometimes combine part of the land in a seascape. But in order to break the barriers, there has to be some obvious rule that marks the division between the two.
A landscape can include part of the coastline, provided the amount of land in the image dominates the scene.
For example, you may see an image taken from a high vantage point of sheep grazing in undulating fields, leading on to a background of steep cliffs and coastline in the distance. That is a landscape that incorporates part of the coast, but you still look at it as a landscape.
The same applies to seascapes; providing the image predominates the sea and its coastline, it doesn't matter if there is a bit of the land included, you can still regard it as a seascape.
Sunsets and sunrises, by the way, do not fall into the category of landscapes or seascapes. They are a breed all their own and they can occur anywhere on land or sea, or even over the rooftops of buildings, for that matter.
If a landscape or seascape is photographed in the early morning or late afternoon, it may include part of the rising or setting sun, but because the image includes only a small part of the sky, it is still accepted as a landscape or seascape image.
Landscapes can also take on the classification of townscape or cityscape. This is when the image is dominated by urban sprawl or high-rise buildings, but if there is only a small amount of these intrusions say, in the background of your image, then it is still classified as a landscape.
Another example, if you were standing on board a ship and you were photographing the sea, if 90 percent of the image was filled with parts of the ship, you couldn't really call it a seascape.
However, like landscapes, seascapes need to include some other elements than just sea. You can imagine standing on a beach taking a shot straight out towards the horizon with nothing else in the image but sea. It would be totally boring and uninteresting. So you need to include some foreground elements as well as a bit of sky as a background.
When composing a full vista landscape or seascape, switch to the Landscape icon on your mode dial or, if you can, manually adjust your camera to Aperture Priority mode and set your aperture for a small aperture, at about f11 or f16. (Small aperture setting - large number). This allows for an image with clear and sharp detail from the immediate foreground through to the distant background. This is a technical requirement of landscape/seascape images when being judged at competition level. Another requirement is that your horizon line, if you include one in the scene, must be level.
Landscapes and seascapes would have to be the most photographed subjects, apart from our kids that is, so we need to know how best to define them in order to present them properly. I hope this has helped.
Please see 'More on Landscapes'
Reference: 'Composition Design'