Would a better question maybe be. Is there historical evidence that the coliseum was ever flooded for water sports?

One important thing to note is that, if it was indeed the Flavian Amphitheatre being flooded for battles (and there's debate over this), it would have taken place before Domitian constructed the hypogeum, the system of underground facilities to support the games going on above. There are certainly no references to naumachia in a flooded arena on the site after Domitian's renovation.