Seawater and salt water

The kidneys regulate how much salt and water stay in the body. They can only concentrate urine up to a certain point. Seawater contains far more salt than the kidneys can excrete without using additional water from the body. Anyone who drinks seawater therefore loses water overall – dehydration gets worse.

This also applies to small amounts and to „stretching“ fresh water with seawater. Every sip brings salt that costs extra water to excrete. The apparent gain in fluid is an illusion.

Something similar applies to urine: it contains salts and waste products that the body was trying to get rid of. It is not a substitute for drinking water, and survival manuals such as the US military manual FM 3-05.70 explicitly warn against both.

Salt is dissolved in the water, meaning it has broken down into individual, tiny particles. That is why it passes through every ordinary filter, whether cloth, sand, ceramic, hollow fibre or activated carbon. Chlorine, chlorine dioxide and UV light kill germs but do not change the salt content. Boiling even increases the salt concentration, because part of the water evaporates.

Only processes that separate water from salt produce fresh water from salt water. One is distillation – with solar heat in a solar still or with a heat source; the other is reverse osmosis, in which water is pressed at high pressure through a very fine membrane. Hand-operated devices for this exist for boats and life rafts; they are used according to the manufacturer's instructions.

A solar still often yields little. With a heat source it is quicker but costs a lot of fuel. Both are supplements to the most important sources at sea and on the coast: water you carry and rain you catch.

Brackish water is a mixture of fresh and salt water. It is found mainly in estuaries, coastal lagoons and marshes, where rivers meet the sea and the tides push salt water inland. It may taste only slightly salty, but it also contains dissolved salt and is therefore just as unsuitable as drinking water.

The salt content can change with the tides: at high tide, salt water reaches further upstream. Fresh water is more likely to be found further upstream, above the tidal influence. A map helps you estimate this zone.

Even real fresh water from a river is raw water and must be treated. Estuaries also often lie downstream of settlements, harbours or industry. So always check what lies upstream as well.

The best strategy is to limit water loss: seek shade, rest instead of working unnecessarily, wear covering clothing and shift exertion to cooler times of day. Every drop of sweat you do not lose need not be replaced.

Catch every rain shower – with tarps, sheets, sails and every clean container you have. Briefly rinse the collecting surface first or let the first drops run off, because it may carry salt crusts. Use carried supplies deliberately instead of holding them back to the point of exhaustion.

If there is a desalination device on board, use it according to the instructions. A solar still can supplement on land. And: anyone in distress at sea or on a remote coast should call for help as early as possible – by radio, phone or the distress signals available.

Do not drink seawater – not in small amounts, not mixed, not boiled. This applies even when thirst is very strong. Urine is not a substitute either.

Warning signs of dehydration are thirst, dark and infrequent urine, headache, fatigue and dizziness. Confusion, severe dizziness or reduced consciousness are serious signs: then medical help is needed as quickly as possible via the local emergency number (112 in the EU). This text is not medical guidance.

Do not rely on myths, such as that you can „get used“ to salt water, or that boiling or an outdoor filter removes the salt. Both are wrong.