Swimming is a good whole body exercise, and is also a good core workout. However, there are many exercises that can benefit your abs, such as crunches, pilates, yoga, etc.
Swimming is a phenomenal exercise because you are forced to work all of your major muscle groups in combination. You can get a cardio workout and you will also build lean muscle.
No, because your abs are meant to support your weight when on land. In the water, they don't do much. The best ab exercises involve weighted loads while standing.
Swimming is a great overall exercise but is not abs specific. Crunches, machines the simulate crunches, are the best exercise for abs. Specifically those exercises that focus on abs so that you can "feel" it.
