Shadow Sealeo

Water Gun + Body Slam and Aurora Beam or Water Pulse

Water Gun has a good/average performance in PvP, while Powder Snow is well known for its fast energy gains. Both are viable depending on how you want to use Sealeo and what you expect from your opponents. It should be noted that while Powder Snow has a below average damage output, it can still pressure certain Pokemon weak to Ice such as Altaria and Flygon. 

In general, Body Slam is almost always desired on Sealeo as it allows Sealeo to either bait shields or annoy the opponent. Or both. Body Slam can also be used to bruise opponents that can deal with Sealeo's Water or Ice attacks, such as Sealeo mirror fights or vs Lapras or Kingdra. The choice between Aurora Beam or Water Pulse is up to preference. Both have some of the poorest performances on paper (both require 60 energy and have a DPE [Damage Per Energy] of 1.3 and 1.2 respectively) and they each fulfill a different need. Aurora Beam does have a perk of dealing with Grass types meant to counter Sealeo, especially if you chose Water Gun as your fast move.