Yes, it is safe to eat. Even when water melon "goes bad," which it almost certainly hasn't yet since it's been in the fridge, it doesn't become unsafe to eat. It only means that it has begun to ferment. When fruit ferments, it doesn't become toxic but starts gaining an alcohol content, so the worst it might do is give you a bit of an alcohol buzz. Unless you taste the distinctive bite of alcohol, though, it hasn't yet begun the process of fermentation. If you keep it in a sealed container, you will know it has started to ferment if the container starts to bloat and/or pressure or gas escapes when you open the container, kind of like when you open a bottle of pop. Anyway, the fermentation process as the natural sugar in the water melon turns into alcohol keeps harmful bacteria from building up because of the presence of alcohol.