A gravity bong isn't really a water bong because the smoke isn't filtered through the water, the water is just there to suck the smoke into the container. Your problem doesn't have anything to do with the water, it's most likely that your taking huge bong rips that's making you sick. Next time you get a chance to smoke out of a bong, take small hits and see if you can handle them fine. If you can, then the reason you are getting sick is definitely because of taking huge hits.