3. Create a new layer on top of the ocean you just "painted". Now comes the fun part...don't worry if you can't draw a straight line, that actually works against you now!.. identify the divisions of light and dark in the ocean, the big ones you created in step one, not the little effects from step 2, its these divisions you are going to focus on. Make a small round brush, about 65% hardness and size 10 or so depends on the pixel scale for your map, adjust accordingly when you see what happens. I set shape dynamics to pen pressure. Scattering is set to 200% with control off. Now choose white color and set brush opacity at 40%. You can do this with a mouse as well, just ignore the shape dynamics and make your brush about half the size, about 5 in my case. Now you are ready to paint waves...scatter them loosely along those divisions of light and dark. Make your waves scallop shapes running parallel to your shorelines, make some short and some long, its ok to be fairly chaotic, it helps, just don't paint over the a wave that is already drawn otherwise you will have an area of double brightness. It is easiest to draw the waves closest to the shore first and radiate outwards, embrace the natural patterns this creates and continue them. Also as you move outwards from land you can drop your opacity down as an option...here I kept my opacity at the 40% all the way, but I did draw the waves more and more loosely as I went outwards. - Erase waves you aren't thrilled with and just keep things fast and loose, the definition will be there eventually.
Addendum: Don't worry about details too much, though if your waves are going to be quite large you can go back in with a smaller brush and add some more splatters to the leading edges.