You’ll be able to stay comfortable when you are out on the water by having enough room to stretch your legs. This will help to improve your circulation and allow you to stay out on the water for much longer.
You will also need to have a decent backrest. If you are going to be spending hours by yourself with nothing but your fishing rod (see the best salmon and steelhead fishing rods here), then having enough back support will be really important for keeping you in good shape.