Should I eat when I don't have anything to drink in survival situations?

In short:

• You can eat foods high in water and low in proteins, such as fruits, vegetables and milk.
• Do not eat foods high in proteins, such as meat, fish, eggs, cheese, nuts and seeds.

PROTEIN IS DEHYDRATING

Every gram of protein you eat results in loss of 8 mL of water with urine (calculated from NAP.edu):

Urea, a major end product of metabolism of dietary proteins and amino acids, requires water for excretion by the kidneys. Renal excretion of 1 g of urea nitrogen (2.2 g of urea) requires 40 to 60 mL of water. Thus, if a person consumes 63 g of protein in a diet that contains 2,100 kcal, the volume of water required increases by 0.4 to 0.6 L/day above the basal osmolar excretory requirement of 0.5 and 0.75 L/day in younger and older individuals, respectively.

METABOLIC WATER

Amount of water produced in your body from macronutrients (Encyclopedia.com):

• 100 g of proteins = 41 mL of water
• 100 g of carbohydrates = 55 mL of water
• 100 g of fats = 107 mL of water

WATER LOSES THROUGH FECES

With stool you lose about 100-200 mL of water per day (NAP.edu)

IN SUMMARY:

• 100 grams of protein results in 758 mL of net water loss (800 mL loss due to urea, 42 mL of gain due to metabolic water).
• 100 grams of carbohydrates results in 60 mL, and 100 grams of fats in 110 grams of net water gain.

When you have no water to drink:

• You can eat foods high in water and low in proteins, such as fruits, vegetables and milk, which usually contain more than 90% of water.
• Do not eat foods high in proteins, such as meat, fish, eggs, cheese, nuts and seeds.

PROTEIN IS DEHYDRATING

Every gram of protein you eat results in loss of 8 mL of water with urine (calculated from NAP.edu):

Urea, a major end product of metabolism of dietary proteins and amino acids, requires water for excretion by the kidneys. Renal excretion of 1 g of urea nitrogen (2.2 g of urea) requires 40 to 60 mL of water. Thus, if a person consumes 63 g of protein in a diet that contains 2,100 kcal, the volume of water required increases by 0.4 to 0.6 L/day.

METABOLIC WATER

Amount of water produced in your body from macronutrients (Encyclopedia.com):

• 100 g of proteins = ~40 mL of water
• 100 g of carbohydrates = ~55 mL of water
• 100 g of fats = ~110 mL of water

You should not rely on metabolic water as a source of water, for example, you should not eat 1 kilogram of sugar and hope you'll get 550 mL of water this way. Sugar gets incorporated into glycogen, which binds some water, which does not contribute to the whole body hydration. On the other hand, carbohydrates can improve water retention (Physiology.org).

In conclusion, carbohydrates and fats, while not necessary hydrating, are also not dehydrating.

WATER LOSS THROUGH FECES

With stool you lose about 100-200 mL of water per day (NAP.edu); this should not discourage you to eat fruits or vegetables, from which you can realistically get, let's say, 2 liters of water per day.

"Water moving to the gut during digestion" should also not be a problem because it is only temporary.