The Undine Chronicles — a series by Quantum Mechanic

Undine was not a new boat. In fact, she had been decades old when, through neglect, she had sunk into a watery grave. She rose from the water to participate in the lives of mere mortals. Unlike her however, she wasn't jealous, requiring only that her consort pay adequate attention to her needs. In return, she provided safety and comfort, and often, healing.