The hydrofoil, carrying 107 passengers and 10 crew, struck the object at 1:15 am today near Hei Ling Chau, a small island in waters on the western side of Hong Kong.
The ferry was towed back to the ferry pier in central Hong Kong.
Three people were in serious condition and 46 were stable in hospitals, said a government spokeswoman, speaking on customary condition of anonymity. The conditions of the others were unclear. Shun Tak Holdings, which owns the TurboJet ferry company, said in a statement that some people were treated at the ferry terminal.
The Marine Department said it was investigating the accident, which has not affected sea traffic in the area.
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The incident comes just over a year after 39 people died and 100 were injured in Hong Kong's biggest maritime tragedy in decades when a commuter ferry collided with a smaller boat.
The tragedy in October last year came as a shock to Hong Kong. The southern Chinese city prides itself on safety and efficiency and has one of Asia's most advanced infrastructures and economies, with first-rate public services.