It does depend on your body composition. The largest energy drain on a person stagnant in water is their body working to keep them warm, and then breathing (lun

It does depend on your body composition. The largest energy drain on a person stagnant in water is their body working to keep them warm, and then breathing (lungs have to expand, muscles have to contract, water has to be displaced). Lying flat, arms out reduces the work needed to stay afloat - but will drain your core temperature faster (although after 27 hours it doesn't really matter, you'll shiver your way to stay warm "enough" or hypothermia will set in). I can't find the study I read after that movie came out a few years ago on this exact scenario. Will reply or edit my post if I can find it.