The tattoo on Enola's back is a map to find Dry Land. The Mariner (Kevin Costner), Helen, Enola, Old Gregor (who was flying the plane) and the Elder escape the Smokers, who get blown up. At the end, everybody but the Mariner stay on Dry Land (Mount Everest?) because he's a mutant with gills and webbed feet and feels more comfortable on the water.
Thanks Lisa E!