They probably tried to stay positive and keep their hopes up. They might have conserved their energy as much as possible.
Survivors in such a situation of being adrift as in the story behind 'Open Water 2: Adrift' would have had to be resourceful. Firstly, they would have had to deal with the physical aspects. They might have made use of any floating debris to create some sort of shelter or shade. Mentally, they would have had to keep themselves occupied. This could be by counting the days or trying to calculate their possible location. Emotionally, they would have leaned on each other. There may have been moments of despair, but they would have had to fight those feelings. Some might have prayed for rescue, while others may have focused on their physical well - being, like doing simple exercises to keep their blood flowing in the limited space they had. All these combined would have been their ways of coping during that harrowing time adrift.