Hersman revealed that there were 14 people in the 22-foot power boat that crashed into the rear of the tug at about 7 p.m. Sunday near Palm Valley in St. Johns County, about 25 miles southeast of Jacksonville. Investigators had earlier said 12 people were aboard the boat.
The NTSB is working with the Florida Fish and Wildlife Conservation Commission, the U.S. Coast Guard and the St. Johns County Sheriff's Office.
NTSB investigators will look over evidence collected by local authorities and interview witnesses and crash survivors.
Hersman said the NTSB doesn't investigate many boating accidents, but said this case with five fatalities and nine injuries met the agency's criteria for a serious accident that warranted special attention.