My blog post about the dire ratings of cruise ships, as reported by Friends of the Earth, has attracted a great deal of interest on FaceBook. People have pointed out that cruise ship passengers don't stop flushing toilets after they get home, and, what's more, they go back to driving their cars. Others have pointed out that whales produce a lot more poop than people do, though whale poop might be a lot better for the planet. Suddenly, swimming doesn't seem all that attractive.
Rick Spilman of Old Salt Blog has produced a great deal of evidence that FoE have their facts wrong, in that many cruise ships have a better sewage treatment system on board than most of the ports can boast -- that when it rains in many civilized places, raw sewage floats down the streams and drains to the sea. He quoted a blog post he had made about it, and here, with his kind permission, it is extracted. (This is the link the entire post.)
As I said, it doesn't really encourage one to dive into the water. Read the rest for an extremely well-argued post.