Fire crews responded to the scene, the Des Moines Marina, about 15 miles south of Seattle, at about midnight Sunday after receiving a report of a boat fire.
Arriving at the scene, firefighters found flames spreading from boat to boat at J Dock along the waterfront.
Firefighters say a 28-year-old woman and her boyfriend, in his 30s, were in the bilge of their boat when there was a sudden flash fire or explosion, burning both victims.
Both were taken to Harborview Medical Center with second- and third-degree burns on their upper bodies, said South King Fire and Rescue Assistant Chief Gordie Olson. Their conditions were not immediately available.
Twelve boats damaged by smoke and flames as the flames spread, and some watercraft floated loose in the marina after the fire burned away their mooring lines.
"I rushed down here and saw the boats flaming, flaming. ... The firemen did their job and put it out," says Del Rivero, who owns a boat at the marina. "The flames, they were shooting 60 feet high through the roof of at the marina, and it's amazing how that fiberglass burned."
Investigators said it is still unclear what specifically caused the fire.
A damage estimate was not immediately available. The vessels were sailboats and power boats 20 to 30 feet long.
The state Department of Ecology says an oil containment boom was placed in the water to contain any oil that may have leaked from the boats.