The Water Signs are called such because they have a cold and wet nature, like the prototypical Water Element. They bear many characteristics in common, such as their nurturing, emotional and imaginative natures. People born into this group tend to be sensitive or even psychic, and are often very magnetic.
Water Sign people tend to be very sensitive to mood altering substances, and generally have more of a tendency to addictive and codependent behaviors than members of the other sign groups.
The Water Signs were originally known as the Cancer Trigon. The three signs belonging to this Trigon are Cancer, Scorpio and Pisces. A Trigon, as defined by the ancients, is a group of signs that share the same elemental nature and are positioned at the angles of an equilateral triangle when they are measured in the circle of the Zodiac.
People born with the Sun in these signs will tend to adapt quickly to their environments, and will usually act in ways that protect them emotionally. Unless they were born at a time during which the planets weighted the horoscope to other Elemental Trigons, people born under the Element of Water have the ability to draw the attention or interest of others without much effort on their parts. They are often hurt easily, but don't tend toward displays of anger.
Every sign of the Zodiac is an individual entity, and has characteristics that it doesn't share with other signs, even the other members of their Elemental Trigon. These differences can be distinguished by the Qualities to which they belong.
Cancer is the Cardinal Sign of the Water Signs, and consequently is the most leadership-oriented sign of the group. A Cancer person has the "common touch" that knows others' needs and how to fulfill them. They are consciously aware of their need for both emotional and material security, and "wear a mask" to protect their tender inner natures. They are often very popular, but have trouble with their cyclically changing moods.
Scorpio is the Fixed Sign of the group, and seeks stability. They tend to keep their intensely passionate natures hidden, are extremely loyal, and are disciplined, courageous and insightful. When angered, they tend to be vengeful.
Pisces is the Mutable Sign of the group, and is very adaptable. Pisces people want to realize their ideals, and are intuitive, dramatic, helpful and wonderful listeners. They tend to have difficulty with finding the stability they need to thrive.
Because of the "perfect" harmony and balance of the equilateral triangle, signs belonging to the same element are considered to work in harmony. Cancer should be a good match for Scorpio or Pisces, but there are so many other factors in the horoscopes of individuals, these archetypal ideas are more like guidelines than laws of nature. The signs of the Moon and the planets at the time of birth can, in extreme cases, leave the Sun's sign as the only factor in the chart representing the Water Element.
Nevertheless, Cancer, Scorpio and Pisces generally do well with one another. Scorpio tends to be quieter and more serious than the others, whereas Pisces is less dependable and values relative truth and imagination. The Cancer will often try to care for someone to the point of possessing or smothering them. These are the most common points that cause members of this Trigon to rub each other the wrong way.
Water signs are security conscious, and cope with it in different ways. The Cancer considers material possessions and others as the source of security, whereas Scorpio finds it in strength, loyalty and the acquisition of power. Pisces seeks security through others, and fears betrayal, although their own loyalties may change from time to time.
They are generally said to be compatible with the Earth Signs, because the element of Earth can contain Water. The ancients defined it a little differently; the Water Signs are cold and wet, while the Earth Signs are cold and dry; they have fundamentally identical and balancing properties.
In other words, Cancer, Scorpio and Pisces usually get along very well with Capricorn, Taurus and Virgo. Again, this is a general idea based on the elemental theme of the Zodiac signs; sometimes Taurus may be annoyed by the never-ending conversations of Pisces, or Scorpio may be ready to take action, but frustrated by Capricorn's patience.
Generally one can say that Water Signs are harmonious, and tend to strike an easy balance with the Earth Signs, when looking at the Sun Sign as a guide to astrological compatibility.