This sounds like overripe watermelon. When a watermelon goes past its prime it becomes mealy, may develop cavities around the seeds and the taste may be unappetizing. Another possible culprit of bad watermelon may be from fungal disease exposure to the plant during growth that can inhibit proper sugar develop in the melon. Eating overripe watermelon should not have any consequences; however, if you purchase a watermelon and find it yucky on the inside return it to the store for a replacement. Have your local grocer help you select a good melon.