Thora and the Green Sea-unicorn: Another Half-mermaid Tale - Tapa dura

When an attempt to steal Shirley, a rare sea-unicorn, results in the sinking of their houseboat, ten-year-old Thora, her mermaid mother, and Mr. Walters stay at the estate of old friends and find a solution to the friends' financial troubles, but the would-be thief is still on their trail.