The girls are worried that they've run out of food and water. They're worried that the sun will go down soon and they'll have to go back to the hotel. They don't know if they're going to make it back to land, but they do know that they won't be able to find their way back. They can't even see the horizon, which means that the distance between them and the land is about 4 to 5 kilometers. It's not like they can see very far, so they have to rely on their wits to see where they are