A huge steam-driven explosion, caused by molten rock mixing with seawater, sent the lava bomb crashing through the roof of the boat and into the vessel’s seating area, authorities said.
The boat returned to port in Hilo on its own power less than an hour later, and three of the injured, including a woman in her 20s whose leg was broken, were taken to a local hospital by ambulance, officials said.
Nine or 10 others with less severe injuries were driven to the hospital by private vehicle, and 10 more people were treated by paramedics at the port.
Civil defence spokeswoman Kelly Wooten put the total number of injured at 23.
The boat was operated by Lava Ocean Tours, one of a number of companies offering daily excursions to passengers who pay to watch the lava flowing into the sea.
The stricken 49-passenger-seat vessel, dubbed Hot Spot, was later seen manoeuvring to its dock after passengers were unloaded.
A photograph posted by the Hawaii Department of Land and Natural Resources showed a gaping hole in its canopy.
Hundreds of homes have been destroyed by the volcano’s eruption, but no fatalities have been reported so far.