My hair now does this without the flatiron! - Waves in 15 minutes! Section hair into big sections then braid each in a loose braid. Run a flatiron over each braid, let them cool down, spray hairspray and undo the braids.
DIY beachy hair: Mix together 2 cups of warm water, and a tablespoon each of sea salt and coconut oil. Spray on wet hair. Section off hair, twist. Then let it dry and shake it out. Always out of styling products so this could be a glorious thing