Authorities said the family, which included two children, was filling up a boat with gas shortly before 6 p.m. and left the engine running.
That created a spark, causing the explosion at Rollingview Marina at 3940 Falls Lake Road.
Mike Driscoll said he and some friends were walking by and heard the explosion. They immediately started helping the family get off the boat. He said at least two family members had burns on their legs.
“One lady had burns all down the front of her leg, probably a half dozen on each leg,” he said.
The family was taken to Duke University Hospital with injuries that did not appear to be life-threatening, authorities said.