Where is the horizon going to be? Divide your canvas into three parts. If your horizon is going to be near the top, you'll have lots of space to play with in the middle ground and fore ground. If your horizon is going to be near the bottom, you'll have more freedom with the background. The decision depends on what you want to focus on in your painting. For example, if you want to focus on the sunset in the background, then shift your horizon downwards.

Having done that, it also pays to pay a little attention to the general mood of the seascape painting. Is it going to be an angry mood? If so, you may want to consider drawing strong waves and apply warm colors such as red and orange. If you're going to go for a peaceful mood, work on the blends to keep the activity level down.

As you can probably tell by now, choosing the right painting style and the right colors will make or break your seascape painting. The color medium that you'll going to work with will also affect the outcome as well.

Since paint seascapes involve describing water, you'll have to choose a medium that allow you to create washes comfortably. Water colors, acrylic paints and oil paints will work create the necessary effects if you know how to work with them. It's also possible to draw a seascape painting using oil pastels but things can get a little messy with oil pastels.

When working with paints, try to work with thin washes so that you can work in layers. You'll most likely have to create a greenish or blueish effect for your waves. Color mixing will do the trick here. Also, a lot of white is required for the foam that the waves create as well as the highlights on the water.

Use a reference photo if you need to. Try it for a few times to get the hang of washes, and you're on your way to creating a great seascape painting.