they capsized
The barges were a last ditch effort to find a Spanish settlement for survival. This is evidenced by their lack of food. Many of the men were near death and unable to help move the barge. When Cabeza de Vaca's men tried to leave Galveston Island on their Barge, the ferocious waves forced water onto the barge causing most of the men to drop their oars. A giant wave capsized their boat, killing three men.
