Both in water
Same-element pairings remove one whole category of misunderstanding. Neither party has to explain why a mood is not an argument, or why something unstated still counted. That saves a great deal of ordinary friction, and it arrives early rather than after years of practice.
What it removes along with the friction is correction. Neither sign will insist on stating the matter plainly, because neither considers plain statement necessary. Assumptions therefore go unchecked, and both signs are capable of nursing an assumption for a very long time.
Cardinal water and fixed water
The modalities do the useful work. Cancer starts and Scorpio holds, so proposals come from one side and endurance from the other. That is a functional arrangement and it accounts for the pairing's reputation for durability.
It also sets the power balance. Fixed outlasts cardinal in any contest of will, and Scorpio does not concede. Cancer's method of getting its way is indirect, and indirect pressure applied to a sign that watches for exactly that goes badly. The trine between them softens this without removing it.
The Moon with Pluto and Mars
Cancer answers to the Moon. Scorpio answers to Pluto in modern practice and to Mars in the traditional scheme. That gives a pairing of the most responsive body in the chart with the two most unrelenting ones.
The Moon registers everything and Pluto refuses to let anything go, which is why grudges in this combination have a long half-life. Mars supplies the sharper edge when a disagreement finally surfaces. The severity depends entirely on the condition of those bodies in the two charts.
Where it grinds
Suspicion. Scorpio tests, sometimes without deciding to, and Cancer feels the test as a withdrawal of trust and reacts by protecting itself. The self-protection then reads to Scorpio as evidence, and the loop closes.
The second point is isolation. Both signs prefer a small world, and neither pushes the other outward. Couples in this pairing often narrow their social life without noticing, and the narrower it gets, the more weight each disagreement has to carry. A pairing with no outside traffic has no third opinion available when the two of them read a situation the same wrong way.
What a sun-sign verdict misses
Everything specific. The Suns describe a general disposition shared by a twelfth of the population, and a relationship is conducted between two particular people rather than between two averages.
Here the Moons matter more than usual, because one of the signs is ruled by the Moon and both are governed by emotional habit. Venus shows how affection is offered, and Mars shows what happens when the water finally boils. Those three are the evidence.
Questions people ask about this
Is Cancer and Scorpio an easy pairing?
Easy to understand each other, yes, since both work below the level of stated things. That ease also means little gets checked or corrected.
Who holds the power here?
Fixed outlasts cardinal, so Scorpio wins contests of will. Cancer's influence is indirect, which is precisely what Scorpio watches for.
Why do grudges last in this combination?
The Moon records the incident and Pluto declines to release it. Neither ruler has a mechanism for letting something fade.
Do I need more than the sun signs?
Yes. The Moons, Venus and Mars across both charts carry far more of the picture than a sun-sign trine does.