Humans can go weeks without food, but only a few days without water. If you find yourself stranded on an island, one of the first things you will need to do for your survival is to find a source of fresh water.
Saltwater sources are not safe to drink, even after purifying. Even fresh water that appears safe needs to be purified because it may contain harmful bacteria, viruses, parasites or chemicals. Drinking water with these pathogens can be harmful, causing illness that leaves you even more dehydrated.
Seek a fresh water source, such as a stream or brook. If you are having trouble locating a source, leave a container out to catch morning dew or rainwater. If you lack a container, curved bark could work as a catcher. You can also dig to find water.
Filter the water through a shirt or sock to remove any sediment. After the water is filtered, you will need to purify it before drinking. The clearer the water, the easier to purify using certain techniques.
Purify water using the sun. The sun's UV rays are powerful enough to purify water. SODIS---solar water disinfection---was introduced 1980 and, in 1991, the Swiss Federal Institute of Science and Technology spread it into the developing world. This low-technology practice can be used if you are stranded on an island.
Ideally, the water should be poured into a plastic bottle and placed on a reflective surface like aluminium. Fill the bottle and set out in the sunlight. Be sure to leave a little room at the top. Water can be purified in six hours on a really hot day, while cooler environments could take up to 48 hours.
Boil water. If you have matches, a lighter or even a survival pack with a magnifying glass, gather some kindling and make a fire. Fill a container with fresh water and bring to a rapid boil. After one to three minutes of a vigorous boil the water should be safe to drink. Let it cool before drinking.
Anytime you are doing something adventurous, bring along some water purifying tablets or matches. You never know when you'll be in a dire situation.
No matter how desperate you become, never drink saltwater. It will make you sick.