I think "Knight" and "Cups" are almost made to be together - because knights must always be in movement, and water that doesn't move stagnates (it is similar, with different qualities, for the Knight of Swords, and of Wands). As a relationship, its quality is going to be changeable, varied, calm and pleasant one moment, and agitated the next. It might feel sometimes like it's disappearing (going underground, like water does), but then suddenly it springs up to life. Like hope, it will spring eternal. Such a relationship never dies. It can't - water is the origin of life. But you can't contain it, either, and you have to "go with the flow" with it!
On the whole, however, it is very pleasant, refreshing and joyous, and it is a relationship that has substance (like water), even if it is shifting.