Dehydration is an illness which causes extreme electrolyte imbalances in the body. It occurs when you do not take in enough fluids to replace what have been lost through sweat and urination. While dehydration is a danger during any sport of physical exertion, it becomes more so during swimming. This is for two reasons, firstly, when you exercise, you sweat. When you are in the water swimming, you do not realize that you are still sweating and thus losing fluid. Secondly, because you are surrounded by water, your brain is tricked into thinking that you have all the fluid you need, and does not signal your mouth and throat to be thirsty. This can be aggravated further if you are swimming in the ocean. As you swim, you undoubtedly swallow salt water which can dehydrate you even faster.
As always, prevention is your first line of defense. As you are swimming, be mindful of your liquid consumption. You will want to avoid alcohol as much as possible as it is a natural diuretic. Fruit juices are not terribly helpful either as they often contain large amounts of sugar. Since your brain will not notify you when you need to drink, make sure that you schedule specific breaks during your swim to drink. You will want to consume at least half a gallon of water each day to prevent problems. This should be increased if you spend longer amounts of time in the water or if the weather is extremely hot. You can also supplement water with sports drinks as they are specially formulated to replace lost electrolytes during physical exertion. These breaks should be scheduled every 15 to 20 minutes during your swim and should allow you to consume 8 ounces of water or sports drink on each break.
If you have already become dehydrated, treatment for mild cases can be administered at home. Instead of downing a great deal of liquid at one sitting, sip slowly or use ice chips. Too much too quickly will cause uncomfortable cramping and pain in your stomach. If your dehydration is severe, medical attention may be necessary as they can administer intravenous fluids to re-hydrate your body.