She made contact with the ship at her intended target and was running low on fuel. It is a small island and there’s not any other land around. Therefore, she came down in the water, since she obviously didn’t land on the island.
Her flight was very poorly planned. She was dead as soon as she took off.
As I understand it she never really understood how to operate the direction finder. She also had issues with receiving radio communication on the leg to Howland, possibly due to the plane’s belly antenna being torn off while taking off from the rough dirt field at Lae (New Guinea).
On top of it all, there was never ending confusion the radio schedule, i.e about the times she would be transmitting and the times she would be listening and the frequencies used by herself and the U.S. Navy.
The truth is she didn’t seem to like to sweat the details and in this case the result was tragedy.
The more you read about this, the less impressed you are with her flying skills.