Roughing up coastlines is one of those things that's really hard to explain but comes with practice, just study the real world and draw stuff and you'll get a feel for the different coast types pretty soon. The Mediterranean is a great area to look at because there's a lot of different terrain types that meet the sea there, so you can sea what the waves and the wind does to different types of coastal landscapes in the same area. I like your island shapes and the colors, nice job on that, it's very harmonious