Because the water in the ocean is salt water. If you have too much salt, it can be poisonous, and with salt water it's pure salt. Also the fact that you need other foods in order to survive.
yes
The main conflict in a story about a person stranded on a desert island would likely be person vs. nature, as the individual struggles to survive in a harsh environment. Additionally, there could be internal conflict as the person faces psychological challenges like loneliness, fear, or despair.
With sufficient nutritious food and plenty of water, a person can survive indefinitely.
Water
A person can survive indefinitely if that person has sufficient food and water.
Interestingly, this was just covered in National Geographic. Their answer was about seven days for a person who was healthy and well-hydrated to start with. This would obviously be reduced if the person is in conditions that would tend to dehydrate them (in the middle of a desert, for example).
use sunscreen
A person can survive for about 3-5 days without any food, but only drinking water can sustain life for about 1-2 weeks, depending on individual health and conditions.
Interestingly, this was just covered in National Geographic. Their answer was about seven days for a person who was healthy and well-hydrated to start with. This would obviously be reduced if the person is in conditions that would tend to dehydrate them (in the middle of a desert, for example).
You could survive in either desert, given the appropriate supplies, clothing and transport.
A person can only survive about a week because every human need water to survive
That would depend upon a number of factors. How serious is the person's illness? Does he have medical care? Does he have sufficient water and food and shelter? If the person does not have a life threatening illness and is living in a town or city in the desert with water and food, that person can survive indefinitely. Thousands upon thousands of people are in that situation everyday.