The Moon is the whole argument
Cancer is the only sign ruled by the Moon, and the Moon is the fastest body in the chart, crossing a sign in roughly two and a half days. It stands for habit, memory, what someone needs in order to feel safe and what they do when they do not get it. For Cancer, that is not one factor among several; it is the sign's governing principle.
Which is why a Cancer comparison that ignores the other person's Moon is not really a comparison. Whether a partner's Moon falls in Aries or in Pisces alters what closeness even means to them, and no sun-sign list has ever asked.
What the traditional shortlist consists of
Scorpio and Pisces are the other water signs, each a third of the circle from Cancer. Virgo and Taurus are earth, each a sixth of the circle away, and earth is said to give water somewhere to collect. That is the entire traditional answer, and it is short because there are only so many places on a wheel of twelve.
The names are downstream of the geometry. Two separations were ruled favourable in antiquity, and every list since has simply reported which signs happen to stand at them from Cancer.
Cardinal water, and what that combination does
Cancer is cardinal, which places it at the start of a season and gives it initiative, though the initiative is exercised sideways. A Cancer does not usually announce a plan; it arranges conditions until the plan becomes the obvious thing to do. Read against fixed Scorpio, the tradition expects depth met with persistence. Read against mutable Pisces, it expects sympathy with no edges.
Neither of those is a prediction. They are descriptions of a likely division of labour, and either party can find the arrangement restful or intolerable depending on things the chart does not record.
Where the useful information sits
The sun contributes one line to a chart that runs to dozens. A comparison worth doing sets both Moons against each other first, then Venus for the form affection takes, Mars for how a disagreement unfolds, and the ascendant for the manner each person leads with. Then it measures the angles between the two charts rather than within either, which is synastry; the synastry calculator runs that from two birth times.
None of it locates a soulmate. The term has no technical meaning here, no aspect signifies it, and Cancer's habit of reading a feeling of familiarity as recognition is a Cancer habit rather than evidence.
Read the pairing you care about
Whatever sign is on your mind, that combination has a page and it says what the two do well and what they wear down. Cancer and Aries, unlisted and awkward, will tell you more than Cancer and Pisces, because it has to account for itself.
Then compare the actual charts if the birth times are available. The result is smaller than the question people bring, and it is at least true.
Questions people ask about this
Which signs are named as good matches for Cancer?
Scorpio and Pisces in water, Virgo and Taurus in earth. Both groupings follow from the angles rather than from any study.
Does astrology recognise soulmates?
No. There is no placement or aspect for it, and the idea comes from outside the tradition.
Why is the Moon central to a Cancer comparison?
The Moon rules Cancer and moves through a sign in about two and a half days, so it varies enormously even among people born the same week.
What is worth comparing between two charts?
Both Moons above all, then Venus, Mars and the ascendants, and the aspects between the charts.