For this tutorial I sketched the examples below using Procreate, using the ‘Narinder Pencil’ in the ‘Sketching’ brush library, on my iPad with my Apple Pencil (1st gen).
This style is the simplest and easiest to draw. Simply add a few, spaced out, short lines following the curves of your coast.
For this second style draw similar length lines along your coast for waves but draw them closer together. You want much smaller gaps between the lines than the previous style.
Add a second layer of lines (step 3), slightly offset from the edges of the first layer. I drew this layer with a smaller size brush with lower opacity in Procreate. You could, on paper, use lighter pressure or a different pencil or pen for these lines instead.
For this style the steps are much the same as style 2, the difference is that you want to draw a continuous line as opposed to multiple lines/waves close together.
Again I used a smaller brush size and a lower brush opacity for the second layer/ (step 3).
For this final style, I drew a slightly more random collection of lines for the waves, still following the shape and line of the coast. I also used a much lighter brush to draw the waves and I gradually reduced the pressure on my pencil the further the lines got from the coast.