River fishermen (especially for rivers with riffle areas) want a boat that turns quickly. Lots of kayaks are made with strong keels that keep the boat going straight even with sloppy paddling technique. However in a river, not only do you need to turn faster (to avoid rocks in the rapid areas), but the current piles against the keel which tends to keep the kayak sideways, making it even harder to turn. A strong keel (or pontoon design) also gives the kayak more initial stability, however it makes it slower to start moving/turning (a disadvantage for river fishing). Both types usually have plenty of secondary stability (ie keeps you from falling all the way over). River kayaks tend have more rocker (bend from bow to stern) as this allows them to slip over bouncy water better (bob like a cork) rather than spear through the water. A design between the two usually is never a "fun" boat to paddle, as they mostly always feel like they are lacking in one aspect or another.
If you ignore the branding, this is a decent description of a kayak attributes (from a sea kayak perspective, but it mostly holds true for other types of kayaks):
http://www.cdkayak.c...sign_basics.php