One of the benefits of doing crunches in the pool is that you won't experience lower back pain caused by this type of exercises when done on hard surfaces.
This exercise is done with a pool noodle or a floating belt, to help you stay afloat. It can significantly strengthen your abdominal muscles if performed regularly.
Start by warming up in the shallow end. First by doing high knees jogging on the spot for 1 minute, then around the pool for 2 minutes.
Put a noodle on your back (or use a floating belt) and lay on your back. Straighten your legs and close your feet together, hip-width apart. Rest your hands on the noodle and relax your shoulders.
Move back to the shallow end by doing slow breaststroke kicks on your back while breathing deeply. Then hold the noodle in front of you and walk around the pool for (1 minute)