Seal is fun and a great abdominal workout. It is a rolling exercise whereby one must find a way to initiate and control the rolling with the abdominal muscles, and not depend on launching oneself back and forth with momentum!
The key to Pilates rolling exercises is using the breath and the core to control the rolling. Read more in Tips for Rolling Exercises.
Sit up tall on your sit bones and bending one knee, then the other, lift your feet and put your arms inside your legs. Wrap your hands under your ankles and grasp the outside. The knees are just outside the shoulder, not too far apart, and the feet are together.
Get your balance here. your core should be working already! Next, the roll.
If you have back or neck issues you should not do the rolling part. However, you will find that taking the preparatory position (above) and holding it will provide a very good abdominal workout.