Investigator Dan Baker of the National Transportation Safety Board says the victim died Friday. Winnebago County Chief Deputy Coroner Chris Shea says the woman was a passenger. Her name was not released.
Baker says a third injured person is out of the hospital. All three are from Minnesota.
Authorities say the aircraft registered to a Minnesota couple was trying to take off at the EAA Seaplane Base on Lake Winnebago
EAA says the amphibious plane -- meaning it can take off and land on both land and water -- apparently hit a wave during takeoff and tipped over.
"He was doing his takeoff at that point, and some occurrence caused the airplane to flip over. Beyond that it really comes down to the investigation," EAA senior communications adviser Dick Knapinski said.
One of the three people on the plane was able to get out when a door opened from the impact, the sheriff's office said. The other two were trapped.
The National Transportation Safety Board is leading the investigation.
Federal investigators are doing an inspection of the plane on the water, then the seaplane will be taken to a hangar.
“The NTSB will talk to not only the person that they can talk to already on this but also to any of the occupants of the aircraft, anybody that may have seen it, and get a look at the aircraft, the weather conditions, pilot skill and so forth," Knapinski said.
The sheriff's office described the water as rough and choppy.
EAA said it's ultimately up to the pilot to decide if conditions are right for takeoff.
"At anytime when flying, the pilot in command will take a look at the conditions and see, 'is it the proper conditions within my airplane's limitations and within my limitations?' and make that go/no go decision according to that," Knapinski said.