Hedge funds and private-equity funds that waded into shipping the last several years, hoping to ride the next boom in a famously boom-and-bust industry, have gotten stung by a delayed recovery in the dry-bulk vessel segment.

On Wednesday, the Baltic Dry Index, the global benchmark for freight rates for ships carrying raw materials, plunged to its all-time low, dealing a fresh hit to hedge funds and private-equity firms that over the last several years had bet the rates would rise.