Result if a nuclear warhead were detonated above a large body of water?

I'm sure this question must seem strange, but I was wondering about the hypothetical result of a nuclear warhead (or other comparably large explosive device) being detonated (in a city or similar) directly above a large body of water, such as a lake or swamp. I'm sure much of the water would be forced out of the lake, but would a considerable amount simply vaporise? I know this is obscure/odd, but that's why I've come here. For a bit of direction, I'm trying to gauge the overall effect of a nuclear blast on a city's water supply. I've been looking at post-apocalyptic culture and scenarios lately and I'm wondering what proportion would remain available (e.g. not vaporised). I realise that most remaining water would be incredibly irradiated - I'm not concerned with this. I just wanted a rough idea of how much would be left to 'salvage', and thought an individual body, feeling the full brunt of the force, would be the best place to start. Thanks.