You can eat watermelon with beer, but alcohol is not recommended.
Beer and watermelon together do not produce additional harmful substances, so they can be paired. But beer is essentially an alcoholic beverage, and drinking alcohol is harmful to your health.
Every 100 milliliters of beer contains 32 calories, of which 0.4 grams of protein and the rest of the calories come from alcohol. Alcohol is a class 1 carcinogen, and long-term alcohol consumption can lead to a range of health risks.
Watermelon itself contains some nutritional value, per 100 grams of watermelon containing 26 calories, of which 5.8 grams of carbohydrates, dietary fiber 0.3 grams, 0.6 grams of protein, 0.1 grams of fat, vitamin E75 micrograms. There are also some other vitamins and minerals.
Although beer and watermelon can be paired, it is recommended that you do not drink beer while eating watermelon sensibly.