If I were lost in the seas similar to Pi, I wouldn’t have to worry about water because I will carry enough water to last for 140 days. Lifejackets would be already available. A whistle could be my choice of item. Even at night if a ship is passing by I can whistle and attract the attention of the ship. When I feel lonely I can whistle. I can scare away prowling sharks too with my whistle as it would sound like the sound made by dolphins in deep waters.
Occasionally fishermen also come deep into the ocean with their trawlers or fishing boats in early evenings or dawn. I could easily invite their attention. Inspite of having a whistle, if no boat or ship comes that side, it would be difficult to survive. But I can whistle as well as wave the orange life jackets to attract possible rescuers.