Are there any legal issues involved with going to a public river and taking pictures of white water rafters coming down the river? Firing them up to, say, a Smugmug Website for sale. More importantly, what if the photos include commercial rafting companies?
I'm going to ramble on here for a little while about a few different things; this isn't directly an answer, so everyone please feel free to vote down at will.
If you do want an answer to your specific question, "Legal Issues Photographing Rafting Companies," the only possible answer is: ask your lawyer.
It's not clear why you're taking photographs of white-water rafters: is it just for a scenic view of a river with rafters on it (as I think @ElendilTheTall assumed in his answer, which covered all the most important issues with its links), or are you taking pictures of the rafters themselves?
I know that many white-water rafting companies try to sell photos to their customers: are you planning on competing with the rafting companies themselves by undercutting their prices? Providing a better product at a better price is a time-honored capitalist tradition, but if you're competing with someone who's making money and you're doing anything that's borderline legal, whichever side of that legal-illegal line you're on, you should probably expect to be challenged (and even if you're doing something legal, that "challenge" could cost you enough in legal fees that it isn't worth fighting).
If you're taking pictures of rafters for their own enjoyment, and the companies they're hiring for their trips aren't already offering pictures for them (or they're not taking quality pictures), have you considered talking to the rafting companies and trying to get a job working for/with them?
How exactly are you planning on selling the photos to the rafters? Are you just going to set your business card on windshields in the parking lot, telling them to visit your website to find and order their pictures? Or are you going to talk to each person and give them a personal login and password? I suspect that most rafting companies that take photos make offers directly to their customers. This does three things: first, it gets rid of any publication issues because they're privately offering a photograph of someone to that person. Second, it means people don't find photographs of themselves publicly posted on the Internet without their permission (I'm thinking of the example where someone wears something they don't wear in everyday life, like a woman wearing a bikini). And third, it means that only the subject of the photo can order it: moving again to the hypothetical picture of a woman in a bikini, are people who don't know her going to be able to order that picture?
TLDR: if taking pictures to sell where people can be identified, you may be entering a legal minefield. Unless you're spoiling for a fight, best to work within existing structures e.g. by contracting with the rafting companies.
There should be no problem taking the photos; assuming you are in the US or Europe there are generally no laws against taking photos on public land. However, if you plan to sell the photographs you should obtain a release from the people in the photograph.