Cattails are definitely effected by the water. I have found they they are only edible during crtain stages and seasons. I can't remember off the top of my head when these are, but you can check either in a book or with some of the more knowledgeable people around here. The problem with cattails when it comes to the water that they are in is that crummy water is crummy water, no matter what you pull out of it. The part of the cattail that you eat is certainly going to be in contact with that water and so the necessary precautions will have to be taken. I've seen some grow in spots where the water was sufficiently clear enough and the source of the water had been tracked down and determined to be safe to drink, so the cattails there could be pulled up and eaten without too much risk. Unfortunately this is usually the opposite of the case. Like I said, I'm sure some more knowing people will jump in here and tell you all about them. That's my basic knowledge. I have eaten a lot of them. Pretty tasty in a pinch. Given how common they are, it's a good wild edible to know and be able to identify.
Edit: Don't eat the whole plant. There are uses for all of the plant, but not all is good to eat.
There are a lot of common plants that, unless you find out, you would never expect to be edible and they are EVERYWHERE. A good example is the green briar. Properly identified, the new growth of green briars is all over the place in Spring and I think also parts of Fall and pretty tasty. Tastes a lot like green beans or something. Usually the new growth that isn't eaten by animals is located higher, so you'll have to pull the briars down that run up in tree branches and see what you can find.
One that I was happy to find around here was Hemlock. Finding one of those trees in Spring is like finding a never ending bowl of popcorn... well popcorn that tastes like lemony pine.
