Water is by far the most important nutrient for pets and humans. Most of the human and animal body is in fact comprised of water. Adequate water ingestion and hydration is critical to maintain adequate circulation, as well as the health of various organs and tissues, including the heart, brain, liver, kidneys, lung, intestines and pancreas. When water consumption is not adequate, or pets lose excessive fluids through vomiting and/or diarrhea, pets may quickly become dehydrated.
While mild dehydration does not usually cause too many clinical problems, severe dehydration may lead to worsening vomiting and/or diarrhea, constipation, as well as progressive weakness, electrolyte imbalances, circulatory shock, and even collapse in severe cases. Urinary tract stones are also more common in pets frequently dehydrated. Organ failure, including kidney failure, may occur. Cats on dry food only diets are particularly prone to dehydration, which is why it is important to include at least some canned food in a cat's diet.
There are many techniques to encourage increased water consumption in pets. I've seen many pets prefer to drink out of water fountains or running tap water, rather than from the water dish.
Particularly in pets on dry food only diets, adding water to the dry food, not only can help with digestion, but also enhances water consumption, as well as prevents dehydration. I have also added small amounts of salt to the diet, which also will encourage water consumption and adequate hydration.
This is especially helpful in multi-pet homes, as well as in cases where one pet needs or drinks more water than other pets in the same household. Using different types of water dishes, from ceramic to metal, may also encourage some pets to consume more water. Including some canned pet food in the diet is also an easy way to increase water consumption.