According to the Humane Society of the United States, guinea pigs enjoy watermelon, but it is best used as a treat. Watermelon, like many fruits, has a great deal of sugar and water.
Guinea pigs have rather sensitive digestive systems, and too much of certain foods results in loose stools or diarrhea. The best food for a guinea pig is high-quality commercial pellets made from timothy hay. The pellets should be fortified with vitamin C, and they should not include artificial colors, sweeteners or fillers.
Fresh timothy hay is also an important dietary staple for guinea pigs. The hay provides fiber, helps control the growth of the guinea pigs' teeth and functions as bedding material. Most washed leafy vegetables are good foods for guinea pigs, and other vegetables provide some variety.