I believe there's is a typo in your post. You said one of Mercury's ruling sign is "Capricorn (air)". It would be Gemini (air), instead. Capricorn is a cardinal earth sign, not an air one. The exalted sign is Aquarius, perfectly in line with Uranus being the higher octave of Mercury.
Planets, elements, and signs are different from each other. They have correlations between each other, sure. Some qualities of Mercury are best expressed through Gemini, some through Virgo.
These are the purest expression for Mercury. Mercury in a sign represents how its energy is expressed.
You can see how Mercury expresses in water signs by reading the descriptions of Mercury in Cancer, Scorpio, and Pisces. Notice, however, that Mercury struggles to express through the medium of water, especially through Pisces. That's why we say Mercury is in detriment when it travels through Pisces.
So, in the Western conception of elements (as well as in the Vedic and Japanese conceptions), water doesn't relate well to Mercury. (Fire even less but you didn't mention it).
The page you linked in Apollo's astrology shows perfectly why Air is the element that relates to Mercury the most. Read all the rulerships of Mercury. It's clear. Even its symbolism. They are all related to air.
It has been said before Chinese elements do not directly relate to Western elements. I don't know if there is such a thing as Chinese astrology with Mercury and our other planets, but that would be based on their folkish conception of elements.
For example, Wood has some overlaps with Akasha (and Chinese Earth has some overlaps too) but it's not strictly one-to-one for everything. There's no Air among the Chinese elements. They have Metal, which has overlaps mainly with Earth but also some with Fire. You should not look at it as a 1-on-1 equivalence.
If you're interested in our astrology, which is very effective, then you should read the whole Apollo's Astrology section, and then about the elements in our meditation section. Then I suggest meditating on what you learned and drawing parallels.
The more you meditate on your chakras, especially the bottom chakras and the middle chakra, the more your understanding of the elements will expand. For example, after breathing energy into them and vibrating your chosen mantra, you can focus on one at a time, and set the intent to observe how the element works within you. This is the quantum/microcosmic level.
Externally, when you reach a certain level of advancement, you can start meditating on the elements in nature, even breathe them in and analyse their properties.
If you're interested in Chinese elements, which are a completely distinct system, I suggest studying Traditional Chinese Medicine. A good book is Foundations of Chinese Medicine by Giovanni Maciocia.
Spiritual understanding is not a left-brain exercise. Instead of trying to categorise things a lot, it would be best to focus on integrating them. The spiritual part of the brain doesn't work by division and linearity.
Active Helper