It sure can get cramped sitting in a kayak for any duration of time, especially in whitewater boats, especially for six-footers. This would be reason enough to stay loose and flexible while kayaking. Of course, comfort is one thing and preventing injuries is another altogether. It is absolutely essential to prevent pulled muscles, dislocated shoulders, and lasting soreness to ensure that you stretch properly for kayaking.
Given that it's winter, its easy to think that you'll worry about stretching for kayaking, when you're kayaking. Flexibility, is however, something that must be a way of life and not a seasonal endeavor. So this winter, while watching TV, before you go to bed, or whenever is convenient for you, determine to develop a stretching routine that will keep you comfortable and safe in your kayak.