In the olden days of astrology, Capricorn and Aquarius shared planetary ruler in the sombre, conservative Saturn. Quite a few astrologers believe that old rulerships still have their significance, and it is reasonable to assume that two signs that seem so different at the surface as Cappy and Aquarius indeed do have more common ground in reality. Aquarius is the reformer and Capricorn the conservator. Aquarius the rebel and Capricorn the authoritarian. Yet both have the capacity to leave their personal views and subjective opinions behind and focus on rationality. Both are sometimes accused of coldness and indifference. Both are detached and objective, yet Capricorn in a practical sense and Aquarius in an idealistic and intellectual manner.
I'm certain these signs can get along well also in a romantic relationship, but it is not likely a union characterized by a lot of heat, passion and romance. If Capricorn can soften up a bit and become a bit less fixed in their manners, and Aquarius can come down to earth and tone down their eccentricity, this can be a successful partnership or at least a long term friendship.