Ducklett evovles at level 35, so is Swanna worth it, and is it worth transferring up to generation 6 (omega ruby)?
No, Swanna is not worth it. It's outclassed in every way by Gyarados, another Water/Flying type that's far easier to get, has higher Attack and a better Ability. The only thing Swanna has over it is its Speed and the ability to learn Fly. You'll have to pay for Bank to get one over as well, when you can get a Magikarp right then and there in Dewford. Gyarados has so many moves that Swanna can't have, like Earthquake and Crunch. Swanna's movepool is pretty pathetic.
Hope I helped!