Your Empoleon would have learned Swords Dance while it was still a Piplup, at level 11. If you made it forget, you can use a Heart Scale at the Move Relearner.
[Edit] Sorry, I was looking at the Gen 6 page when I wrote this. Prior to Gen 6, Piplup does not learn Swords Dance from level up. However, it did learn it from TM. My bad, good catch PX. [/Edit]
For Water STAB, you can choose between Aqua Jet for priority or Waterfall.
For Steel STAB, your options are a bit narrower. You will probably want Steel Wing, but Metal Claw is still an option (if weaker).
Keeping Swords Dance, you only have 1 slot for coverage. Drill Peck is certainly an option. You may also want to consider Earthquake.