Speed and whitewater
From KayakWiki
When kayaking in white water, the goal is control and not usually speed. Having a long round bottom boat can get you down the river in a hurry provided there is nothing in your way; it is a little like wrapping your legs around a tree trunk and heading down river. Most whitewater kayakers usually go for the shortest boat that they can fit into. It is critical to be able to turn a kayak in white water as quickly as possible on all three axes. Turning the bow left and right are the most common but it is equally important to rotate the boat onto its edges so that they can dig in and carve through the turns.
Exceptions exist for slalom and wildwater racing. Both those sports use kayaks with longer waterline lengths. Downriver kayaks also have stiffer tracking to enhance forward speed over maneuverability.

