Swanna or Basculin are your best bets. You say you knocked out the Ducklett on Driftveil Drawbridge, but you can in fact encounter more. Swanna can learn Fly as well, making it useful in multiple aspects.
Basculin can be found in almost any body of water. It can learn Cut, Dive, and Waterfall as well as Surf, making it a very useful HM slave.
Hope I helped!