Sirens and Water Nymphs

Greek mythology presents a number of female water spirits, such as sirens, nymphs, and mermaids. 

Arthur Prince Spear (1879-1959)

In an era when the undersea world was still unexplored and wrapped in mystery, artists portrayed the female nude in settings that were evocative and romantic.

Oceanides by Gustave Doré

The siren was a dangerous but enchanting femme fatale whose beautiful song lured mariners to their deaths.  But other female water spirits were regarded as helpful to sailors.

Hugo Hoppener, Water Nymphs

According to Wikipedia, water nymphs or "Naiads were associated with fresh water, as the Oceanids were with saltwater and the Nereids specifically with the Mediterranean." 

Henrietta Rae, Hylas and the Water Nymphs

"The ancient Greeks thought of the world's waters as all one system, which percolated in from the sea in deep cavernous spaces within the earth." So, to the Greeks, the fresh-water and salt-water nymphs overlapped.

Andrew Loomis, mermaid

The general public in the 20th century is less familiar with the details of Greek mythology, so artists  can't count on people knowing the myths that were taken for granted in centuries past. 

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You can find other paintings of female water spirits in the work of John William Waterhouse, Herbert Draper, and Howard Pyle, who I've talked about before.