Water softness or hardness is basically important for sake.
Iron is the most undesireble content of the water, it spoils color, aroma and flavour, and soft water usually contains less iron (calsium and magnesium occurs together with other metals).
Magnesium enhances the metabolism of the yeast.
Potassium and phosphorus helps the healthy growth of yeast and koji.

Water matters for stable fermentation, miya-mizu is one famous example of rich in mineral water.
Because of healthy and fast fermentation floral ginjo aromas are hardly appears to the sake of Nada area.

Rice takes time to break up the fermentation, this waiting time can leads to contamination.
To avoid this, using so-haze koji is one option to break up the rice rapidly.
Alternatively Controlling the mineral content of the water, especially for ginjo style with tsuki-haze koji, in Saijo(Hiroshima) area.

The yeast for sake starts to activate from low temperature around 6c, and alcohol level reaches around 20-22 in maximum.
Traditionally brewers were using the natural yeast they had in their brewery, but it had the risk of contamination.
Brewing society has started to isolate and distribute the stable yeast, each of them keeps the characteristics of the style. 
Stable fermentation is important not to lose the alcoholic tax revenue for the government.
・No. 6-7: Produce more acidity and muted aroma, mainly used for futsu-shu, honjozo, and junmai.
・No. 9-10: Produce high level of acidity but not more than 6-7, ginjo aroma. 
・No. 14-1801: 'modern yeasts', produce low acidity and ginjo aroma.
・No. n01: Low form yeasts. Avoid the risk of overfloat, easier to clean and avoid contamination.  

Ambient yeast: to make the product more original.
Proprietary yeast: isolate the yeast from flowers or trees, for more unique expression.
Prefectual and regional yeasts: ex. very soft isoamyl acetate banana aroma of Shizuoka, very successful ginjo yeast of Akita and Nagano.

Blending yeasts
・ yeast strain
・liquid after fermentation