Foods To Eat
A study has shown that watermelon contains around 92 per cent of water per 100 gm, along with natural antioxidants such as lycopene, citrulline and ascorbic acid; vitamins such as Vitamin B1 and B6, and minerals such as magnesium and potassium. The presence of high water content and mineral in watermelon may help balance the electrolytes in the body and prevent dehydration. 
Water content in watermelon according to USDA: 91.45 g (100 g)
Cantaloupe contains around 90-99 per cent of water content. Additionally, the peels and seeds of cantaloupe are also rich in polyphenols, flavonoids, tannins and carotenoids that provide a plethora of health benefits to the body, along with keeping aside the risk of dehydration. Cantaloupe also contains electrolyte minerals such as calcium and potassium that may help balance electrolytes of the body. 
Water content in cantaloupe according to USDA: 90.15 g (100 g)
Lettuce is one of the green veggies that is packed with a high amount of water, along with fibre and other nutrients. This veggie makes for an excellent source to beat the heat of the summer and prevent dehydration while fasting. Lettuce is also known to prevent dry skin and protect against the sun.
Water content in lettuce according to USDA: 94.61 g (100 g)
Cucumber is a good source of potassium, folic acid and vitamin C. According to a study, cucumber is good for moisturising the skin and hydrating the body. It also contains the important mineral silica which is a vital component of connective tissues such as tendons, muscles, bone and ligaments. Cucumber can help prevent dehydration during fasting due to its high water content. 
Water content in cucumber according to USDA: 95.23 g (100 g)
Tomato is packed with high content of water and helps prevent dehydration. It makes for excellent summer food and can be consumed during the month of Ramadan to hydrate the body and keep it healthy. The important antioxidant lycopene in tomato also helps maintain multiple body functions.
Water content in tomato according to USDA: 94.78 g (100 g)
6. Skim milk
A study has mentioned that skim milk is more hydrating compared to water as the prior is retained by the body for a longer time than the latter. Retention of this fluid in the body for a long time helps maintain the water levels of the body and prevents dehydration, especially in summer. Therefore, if you are fasting, skim milk will make for the best food. 
Water content in skim milk according to USDA: 90.8 g (100 g)
Foods To Avoid
Alcohol or alcoholic beverages such as beer and spirits act as natural diuretics. Meaning, they contain a substance that promotes diuresis, including forced diuresis, by increasing the production of urine. When urine production increases, the risk of dehydration also increases. Therefore, alcohol must be avoided when you are fasting, also considering that it is forbidden during the holy month of Ramadan. 
Caffeinated drinks such as coffee and tea, when consumed in high amount, can cause dehydration. This is because caffeine plays the role of adenosine receptor antagonist and reduces the reabsorption of sodium by the body cells, which is an essential electrolyte. In a moderate amount, caffeine may not dehydrate, but in a large dose (greater than 500 mg), it can behave as a diuretic and cause dehydration. 
3. High-calorie sports drink
Individuals who are related to sports and are observing fasting during the month of Ramadan are often recommended sports drinks. These sports drinks often have higher glucose content that may help maintain the energy levels of the athletes for a prolonged period. However, in some individuals like diabetics, high calories can increase the sugar levels in the body and cause dehydration. 
4. Artificial fruit juices
Drinking homemade fruit juices without sugar can help treat dehydration up to a certain extent, however, synthetic fluids such as artificial fruit juices may give you instant energy, but cause dehydration due to the high sugar content in them. Also, due to the low sodium and potassium content in them, they may cause stomach problems.
5. High salt foods
Consuming too many salty foods such as chips, sauce and processed meats can pull water from the body cells and cause dehydration. Also, as the frequency of sweating is more during the summer season, excessive sodium and water could be lost from the body. Avoid salty foods to prevent dehydration, however, don’t avoid all as sodium is also necessary to restore total body water and fluid-electrolyte balance.