water
In addition to the idioms beginning with water, also see above water; backwater; blood is thicker than water; blow out (of the water); come on in (the water's fine); dead in the water; fish in troubled waters; fish out of water; head above water; hell or high water; high-water mark; hold water; hot water; in deep (water); keep one's head (above water); like water off a duck's back; make one's mouth water; muddy the waters; of the first water; pour cold water on; pour oil on troubled waters; still waters run deep; take to (like a duck to water); throw out the baby with the bath water; tread water; you can lead a horse to water.