Hmm.. Maybe the translation affects the meaning indeed.
To clarify, McMahon was visiting a city crippled by flood and didn't find anything better to say than "so much water!". The follow up by the French official was "And there's more, you've only seen the top of it!", meaning the surface of the water (implying "beneath the surface, there's even more water, Mr. President, you'd be surprised!").
It's my understanding that the official was somehow trolling McMahon for his.. mmh.. stupid comment.