Kilauea has shown no signs of quieting since it first began erupting on May 3.
One woman's leg was broken when the "lava bomb," which spattered out of the water with explosive force as molten rock poured into the ocean, crashed through the roof of the boat into the vessel's seating area, said Hawaii County Fire Department Battalion Chief Darwin Okinaka.
The boat was able to return to its port in Hilo less than an hour later, and three of the injured were taken to a local hospital by ambulance, Okinaka 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, he told Reuters by telephone.
The woman with the broken leg is her 20s and was listed in serious condition. Other tourists hurt suffered minor burns and other more superficial injuries, he said.
A civil defence spokeswoman, Kelly Wooten, put the total number of injuries at 23.
The Hawaii Department of Land and Natural Resources (DLNR) said the boat was operated by Lava Ocean Tours, one of at least three companies offering daily excursions to passengers who pay about $US220 ($NZ324) per person to watch from a vessel lava flowing into the sea.
Officials for the company were not immediately available for comment.
State and local police and the DLNR were investigating the incident, which happened shortly after dawn near the eastern-most edge of the Big Island, where lava from the 10-week-old Kilauea eruption has been flowing into the Pacific since early June.
The boat's distance from shore and from the site of the lava explosion was not immediately known, fire and civil defence authorities said.
Two entire housing developments consisting of hundreds of dwellings were destroyed several weeks ago as lava spewing from a fissure on the slope of the volcano inundated the Kapoho area. Hundreds more homes have been swallowed closer to the eruption site itself.
The property losses marked the most destructive eruption of Kilauea in modern times, but no fatalities have been reported.
The most serious previous injury was of a man whose leg was shattered in late May when he was hit by molten lava ejected by a volcanic fissure while he was standing on the third-floor balcony of his home about 180 metres from a flow in the Pahoa area.
Lava tourism has long existed on Big Island. Visitor numbers spike each time Kilauea, which has erupted almost continuously since 1983, sends a tongue of lava toward the ocean. The current eruption is one of the longest and most intense on record.
People can only witness the lava from the air or the sea. Boat tours cost about $US220 per person, while helicopter tours start at around $US300 per seat.
All lava flow hiking tours have been stopped, and the Hawaii Volcanoes National Park, where Kilauea is located, is closed to visitors.
More than 80 people, some of them locals, have been cited for loitering in lava zones and face penalties of up to $US5000 and a one-year jail sentence.
