Thanks both.

@Mouse - This seems to be what I was looking for. I was sat thinking 'Why can't I just colour the ocean without it colouring the continents too!' Of course, in vector the ocean is a rectangle placed behind everything, so the whole thing gets coloured. Now I need to figure out which boolean operation to use....

@Pananacakes - Thanks, this is useful to know! For my current map, my problem seems to be that I'm thinking in pixel-mode, but using vectors.