I just started a new Survival map a few minutes ago. It's a good sized swamp, try this one out. You spawn right at the swamp. I haven't looked around much, but it looks pretty big.
Seed # -6444095665036038638
Also, you just have to spawn a witch if you want one. Go into your creative inventory and search "spawn" and click on the Witch.
Thanks,
DrewTheFever