You can't prevent getting stretch marks. Its either you get them or you dont. Once you get stretch marks you can only reduce the appearance of them.
Needless to say, I bought some Cocos butter immediately and now slather it in every night.
Prenatal yoga is fun (: Other than that, walking and swimming like you mentioned. How much activity you can do all depends how active you were before. And cocoa butter for the stretch marks ! Start using it right away, because I didn't think it was necessary until I was bigger and then around 17 Weeks I woke up with massive stretch marks down my side. Needle