The basic premise of a person surviving at sea on a lifeboat is based on the true story. It was a real - life incident that gave the author the idea for this part of the plot.
Well, the idea of being adrift at sea for a long time is from the true story. Also, the struggle for survival against the elements, like dealing with the sun, the sea, and lack of food and water, has its basis in the real - life account that inspired the book. However, the more detailed interactions with the tiger are likely more of the author's creative addition.