Her answer to the splashing-water issue: "There's always a slight risk, because you might get one of the bugs on you. And you might be in a place where there are warnings (about waterborne illnesses) and you'd want to heed those warnings. But otherwise, there's no reason to be so careful that you wouldn't splash your face with water on a hot day."
Swimming, of course, is a different matter. Most parasites and viruses can't enter through just any ol' bodily orifice they need to go through your mouth or nose and you might always inadvertently swallow some water that contains giardia cysts or cryptosporidium. Hepatitis A is also a risk, albeit a very slight one. On balance, I've read of very few swimming-related infestations, especially in the United States.
As for drinking water, says Lange, filtering should be adequate in most backcountry areas where there's not a ton of human traffic. That would, for instance, include just about all of the Pacific Northwest. In some areasalong the Appalachian Trail, for instance, or when drawing water from ponds or streams in populated areas, then the extra step of using a virus killer may be advised.
Generally, use your judgment. If the water looks reasonably clean, and is in an area that's not heavily traveled, then you should be OK. Otherwise, hey, man, buck up! Life is full of risks, you know? I've had giardia myself, and it's not the worst thing in the world. Two people here in Washington drowned just a few weeks ago, so if you really want to reduce your water-sport risks, wear a PFD and stay within yourself. Or, take up a dry-land sport. Maybe best to wrap yourself in thick cotton padding, sit quietly in a concrete-walled room, and read the Gear Guy on your PC screen. But then again, just thinking of all those shrieking, dying brain cells when they're subjected to that....