Yes, as AlexSchacher sais, it could be done digitally – or you could paint some thin yellow-brown watercolor over the map. Provided you used waterproof ink of course! I destroyed a map once because the pen I used claimed to be waterproof but actually wasn't at all, so make sure to test it if you haven't done it already. If you choose to add some color, using for example a lighter tone (=thinner paint, if you do it by hand) on the ocean would look nice and add some further clarity to the map.