According to Y-NotBunnies.com, watermelon rinds are safe for rabbits to eat. However, watermelon and watermelon rinds should only be given to rabbits as an occasional treat. Too much watermelon can cause stomach upset due to the high amounts of sugar found in the fruit.
Watermelon is also full of vitamins and minerals, such as vitamin A, vitamin B, vitamin C, magnesium and potassium, which makes it a relatively nutritious treat to give to rabbits. Typically, it is best to provide rabbits with a 1-inch cube of watermelon flesh or watermelon rind every one to two weeks as part of the rabbit's regular diet.