This is because ocean water contain more salt and by drinking a water with too much salt in a desert you will get severely dehydrated.
Dehydration occurs because a salty water will be hypertonic to your body cells. Hypertonic solution contain more solute relative to that in the cell. When a cell is placed in a hypertonic solution it will lose water to its environment by osmosis. This will cause the cells to shrink and may die of dehydration.
