A couple of things I have found in relation to some of the above responses:
Yes, and SOT can be wet; it's also much easier to get in and out of, stand up on, or sit sideways on. I can hang my feet over one side and recline against the other and fish a drift very comfortably. If I have to take a leak, I can stand in calm conditions, or if I can find water less than waist deep I can hop out and get back in very easily. I can also anchor to fish a flat or shallow rocky section very easily. There have been many times that I have pushed the yak up on the rocks on the Suskie and fished on foot in areas where virtually no one else can reach. As far as the wetness goes, I wear breathable waders with a dry jacket over the top and a life vest to cap things off. I've ditched on purpose to test this system and it keeps me very dry.
I think the big yaks (14 feet plus) should be limited to very large water as they are tough to control in current or limited space. I think a 12 footer is perfect for the Susquehanna. I have paddled mine from the mouth of the Conodiguniet (sp?) almost to the I81 bridge and drifted back. There are some killer smallies in that stretch. FWIW, I am not keen about taking an SOT through any substantial rapids. This is definitely the place for small SIS whitewater yaks.
If you are looking for a more "hands free" system, the Hobie kayaks are worth considering. While initially considered a novelty, they have come to be considered a serious option. For those not familiar, these are kayaks that you peddle like a bicycle rather than paddle like a regular yak. They are great if you are looking for a fishing yak, although they are a bit pricey. As a former racing cyclist, the pedaling really appeals to me. Could be my next yak.