Cruise ship hot tubs are unsavory places at the best of times – depending on the cruise ship, of course. But in case you needed another reason to be turned off them, they’ve just been blamed for two outbreaks of severe pneumonia. More like floating cities than actual boats, a single cruise ship can emit as much carbon dioxide as a million cars in a day. They also pump out ghastly quantities of sulfur dioxide, nitrogen oxide, and particulate matter, all of which are damaging to the health of ecosystems and humans alike. And yet they continue to ply the world’s waters, in some cases for up to ni…