I have found diptychs to be a great format for the seascapes and coastlines. The interest area is where the sea meets the land, and to get as much as possible of that part, you want a painting that is wider than the standard canvas. Also, they are easier to pack than a single canvas of the same size. All the diptychs are made from two canvases each measuring 24" x 30". Here are a few.
I won the Carmichael Prize for 'Best Painting by an Established Artist' for Carlisle Bay August Diptych in 2012.