I ended up using thick off-white card and drawing the details on with coloured Sharpies. I then made up washes of colour using transparent inks so the details showed through. I wasn't completely happy with the way the water had overlays of the ink which caused darker patches, but it was difficult to keep the colour flowing around my wobbly coastlines.
To make the grid, I bought a cheap acetate fronted picture frame in Hobbycraft and drew the grid on the acetate. I drew the letters and numbers on backwards so that the grid and writing would be on the side of the acetate that the map was on, so if people touched the acetate they didn't rub any of the ink off - I used a Sharpie again to draw on the acetate. If we decide to do another treasure hunt the map can be taken out and a new one placed under the grid.
The second game is not quite finished, but I will post a picture when it's done.