I'd say no, but if you were creating some kind of "Water World", I'd expect islands and even better, structures sticking out of the water with new shorelines beyond that. Not that a world couldn't be completely covered in water following a catastrophe, but then the point of the challenge idea is to show evidence of great change. A plain blue map of water only shows water, not the change. Another possibility is to have an underwater map showing city ruins and such surrounding tiny island (former mountain top), etc.

You'd need more than a blue map to show evidence of destruction.

GP