Grab a medicine ball, dumbbell, or weight plate and sit on the floor with your hips and knees bent 90 degrees. Hold the weight straight out in front of you and keep your back straight, your torso should be at about 45 degrees to the floor. Explosively twist your torso as far as you can to the left, and then reverse the motion, twisting as far as you can to the right. The strength you need to paddle a raft is generated in your core, not your arms or shoulders. This is great for your obliques as well as six pack abs.
ABS EXERCISES: T-CROSS SIT UP
Begin lying on the floor with your legs straight and arms out wide to the sides, making a T with your body. In a slow and controlled motion, sit up, lifting your right leg and twisting to the right to bring your left hand to your right toes. Hold this position for a moment, pulling your abs toward your spine. Slowly roll down to return to the starting position to complete one rep. Repeat on the other side.