Sukey and the Mermaid by Robert San Souci
Sukey's new step-pa is a mean, bossy man. Every day Sukey wakes at dawn to work in the garden. All her step-pa ever does is watch her and yell if she so much as stops to fan herself. Sukey's ma calls him Mister Jones. Sukey prefers the name "Mister Hard-Times".
One day, Sukey runs away to her secret place by the ocean. There, she calls up Mama Jo, a beautiful black mermaid. Mama Jo's got a surprise for Sukey; a magical kingdom beneath the sea without time or pain. But it's also without people. Is it really better than the world above?