The signs Cancer, Scorpio, and Pisces are in the water element, and are known as the water signs.

Cancer is the 4th sign in the standard zodiac sequence, and starts at 90 degrees of the great circle. Scorpio, the 8th sign, starts at 210 degrees, and Pisces, the last and 12th sign in the zodiac, starts at 330 degrees. Like all the other elements, the signs in water are in trine to each other, being 120 degrees apart.
For people with planets in water signs, establishing and maintaining emotional security is a primary consideration in life. They need to know that their feelings are acknowledged, valued, and understood. Creating and holding a safe emotional space is paramount – they can be feel completely out of sorts in environments that do not have healthy emotional outlets.
People with emphasis in water can be very sensitive, and as a consequence their feelings can be easily hurt. Others may not be as tuned in to their emotional state as they might want, which might make them retreat into their shell. Privacy and alone time very much needed, so they can periodically rejuvenate and recharge before going out and meeting the world again.
People with Sun or Moon in a water sign experience the world primarily through feelings. Other people and situations create an emotional push and pull with which they need to negotiate on a daily basis. Caring for others emotionally, and finding reciprocation for this kind of caring, becomes a way of life.
To obtain the emotional security they need, they might even take the extreme step of living alone, in seclusion, if it comes to that. There may be a conflict between protecting their feelings, and exposing one’s vulnerability by sharing their feelings with others. This constant tug or war between interacting and retreating may make them appear moody, as they try to process the swell of emotions that arise from their rich inner world.
Mercury in water interprets the world in emotional terms. Words and speech are analyzed for the feeling they convey, or the emotions that power them. People with Mercury in water are gifted with the ability to read non-verbal cues, and can instantly tell WHEN someone does not mean what they say. The imaginative gift of the water signs may be constructively filtered through the conscious faculty of Mercury toward compelling narration or story telling. This helps the water Mercury to cut through cultural and social barriers, and connect with others as simply humans.
People with Venus in the water element need to bond with others emotionally. This is as true of friendships as for romantic relationships. As parents, they raise their children with much emotional give and take. This is in contrast to, say, Venus in earth parents who would be much more inclined to show their love by providing material security. At the same time, Venus in the water element can be big on emotional self-sufficiency, practicing it on themselves and expecting it from others.
Mars in water is a challenging placement. The energy and drive of Mars might be slowed down or cooled off in water. So a heated up Mars in water can fume, or plot revenge, or shrug off the anger altogether–in other words, anything but immediate direct action. People with Mars in water may literally expend energy in or around water such as swimming, or sailing. Or the Mars energy may be applied in pursuits that involve emotional exchange, such as therapy or counseling that plumbs the inner life.
See Horoscopes to find out which of your planets may be in water signs.