For Jupiter, there's also the Jupiter Oak Evolution attack, and the fact she likes plants. And Mercury isn't just blue, her attacks are clearly water based. The odd one out is Venus, who also is the only one who doesn't have the planet's kanji in her name (because her character was created first, as Sailor V, before Takeuchi came up with the theme), but at least she has the Love Me Chain, which looks like it's made of metal.
For Saturn, you might think of the agricultural aspects of the death-and-rebirth theme, or the fact that she's powerful enough to destroy the Earth.
One kanji can have multiple pronunciations which generally carry different meanings. It is necessary to memorize the pronunciation of a kanji in different combinations.
The kanji for water 水 can be read as すい or みず and in different situations these two pronunciations can be changed, e.g. 洪水（こうずい） 水無月（みなづき） 速水（はやみ）