Clean natural salt water can be collected locally in many parts of New Zealand; you just need to ensure it is clean. Collecting about half an hour before high tide is best but not after any recent rain as this can include contaminants from land runoff. Near river mouths or areas where there is a lot of boat activity should be avoided as well for the same reasons. Collected water can be stored indefinitely in sealed containers away from any light sources. If you are worried about parasites (