Does eating fruit make you fat? like watermelon?

No: Some nutritionists recommend 5 servings of fruit per day. Remember everything in moderation, don't eat the whole watermelon by yourself in one setting...

Fruit, like all: foods, has calories. If you eat more calories than you burn off, you will gain weight and possibly become fat. It's 3500 calories to the pound. There are no magic weight gain or weight loss foods, it's all calories and energy in vs. energy out.