Can she drown a dragon?
I really dont think she can do anything besides being blown away if she tries to.

There have been a lot of not so subtle hints that there's more to Juvia than meets the eye. There's a lot of fan speculation that she might actually be a Water Elemental with a human form rather than vise versa.
Does sound like that isnt anything but speculation, and the Water elemental part is more or less proven wrong by her flashback in the guild war arc.