Yep, swimming is good.
Also, there is one my P.E. teacher taught me
Lay on the floor so your face is facing the celing and your heels, bum and neck and touching the floor
Then lift your feet up about 10 cm above the floor keeping your legs straight and the rest of your body still.
So you legs should be rasied slightly too.
Keep your feet off the floor for as long as possible keeping your legs straight.
I really hope I've explained that alright.