Crews responded to the scene near Purisima Road at 6:23 a.m. and found the water tender on its side, with a man stuck inside and another one outside the vehicle, authorities said.
The passenger of the vehicle, a 38-year-old male Ventura County firefighter, was pronounced dead, Santa Barbara County Fire Capt. Dave Zaniboni said.
The driver of the vehicle was outside of the water tender complaining of moderate injuries and was taken to the hospital by ground ambulance, he said.
The water tender was assigned to the 12,000-acre Canyon Fire on Vandenberg Air Force Base. It was unknown whether the water tender was full, and whether it was heading to or from the incident, Zaniboni said.
The CHP is investigating the rollover. No other vehicles were involved.
There were representatives from the Ventura County Fire Department at the scene, who were also assigned to the Canyon Fire, Zaniboni said.
Road detours were in place while the incident was being investigated and the wrecked vehicle was removed, according to the CHP.
The Lompoc City Fire Department also responded to the scene of the rollover.