I try to stay away from clouds layed down with a heavy hand. Use them very subtle and try to work out the sea with a brush. I tend to use a grunge brush with different shape dynamics. There is a tutorial by tear ( saderan if i remember correctly ) that helps you to create very beautiful oceans. But it takes some time and a tablet is useful ( that can be said in general , if you are into mapping and drawing get one, it´s a different world )