Mars was full of water 4 billion years ago. It lost it all to solar winds. Despite Earths strong magnetic field it is still loosing ~25k liters per day (in the form of atmosphere loss). So, yeah, one day there will no longer be surface water on Earth. But the atmosphere will go before that so it will cease to be 'livable' long before that happens.One thing I always wonder with these sorts of post-apocalyptic settings that have the oceans dried up is; How? Where did the water GO? When a body of water dries up the water doesn't cease to exist, it just moves somewhere else.
Granted, this story has a flying, immortal mermaid and could just say that the ocean was magiced away, but that excuse doesn't help the non-magic post-apocalypses.