[Therion] How to draw flooded passages

Please ignore my ignorance. I'm trying to draw cave passage which are mainly flooded but have a few places where you can surface due to air filed domes. See http://static.23.nu/md/Pictures/ZZ6E08C628.png - the orange parts are the ones where we can breathe air. How do I draw something like this? Two layers, or an area where we can surface? If so which kind of area is preferred? Regards Maximillian Dornseif