Water is your most important need. With it alone, you can live for 10 days or longer, depending on your will to live. When drinking water, moisten your lips, tongue and throat before swallowing.
Short Water Rations
When you have a limited water supply and you can't replace it by chemical or mechanical means, use the water efficiently. Protect freshwater supplies from seawater contamination. Keep your body well shaded, both from overhead sun and from reflection off the sea surface. Allow ventilation of air; dampen your clothes during the hottest part of the day. Do not exert yourself. Relax and sleep when possible. Fix your daily water ration after considering the amount of water you have, the output of solar stills and desalting kit, and the number and physical condition of your party.
If you don't have water, don't eat. If your water ration is 2 liters or more per day, eat any part of your ration or any additional food that you may catch, such as birds, fish and shrimp. The life raft's motion and anxiety may cause nausea. If you eat when nauseated, you may lose your food immediately. If nauseated, rest and relax as much as you can and take only water.
To reduce your loss of water through perspiration, soak your clothes in the sea and wring them out before putting them on again. Don't overdo this during hot days when no canopy or sun shield is available. This is a trade-off between cooling and saltwater boils and rashes that will result. Be careful not to get the bottom of the raft wet.
Watch the clouds and be ready for any chance of showers. Keep a tarpaulin handy for catching water. If it is encrusted with dried salt, wash it in seawater. Normally, a small amount of seawater mixed with rain will hardly be noticeable and will not cause any physical reaction. In rough seas you cannot get uncontaminated fresh water.
At night, secure the tarpaulin like a sunshade and turn up its edges to collect dew. It is also possible to collect dew along the sides of the raft using a sponge or cloth. When it rains, drink as much as you can hold.
When desalting kits are available in addition to solar stills, use them only for immediate water needs or during long overcast periods when you cannot use solar stills. In any event, keep desalting kits and emergency water stores for periods when you cannot use solar stills or catch rainwater.
Water From Fish
Drink the aqueous fluid found along the spine and in the eyes of large fish. Carefully cut the fish in half to get the fluid along the spine and suck the eye. If you are so short of water that you need to do this, then do not drink any of the other body fluids. These other fluids are rich in protein and fat, and will use up more of your reserve water in digestion than they supply.
In arctic waters, use old sea ice for water. This ice is bluish, has rounded comers and splinters easily. It is nearly free of salt. New ice is gray, milky, hard and salty. Water from icebergs is fresh but icebergs are dangerous to approach. Use them as a source of water only in emergencies.
Remember, do not drink seawater. Do not drink urine. Do not drink alcohol. Do not smoke. Do not eat, unless water is available.
Sleep and rest are the best ways of enduring periods of reduced water and food intake. However, make sure that you have enough shade when napping during the day. If the sea is rough, tie yourself to the raft, close any cover and ride out the storm as best you can. "Relax" is the keyword - at least try to relax.
Information courtesy of the U.S. Army Survival Manual