Dewgong

Once upon a time, Dewgong was a "decent" Ice Type, capable of acting well enough in an infant meta that loved Ice Types and had little to fill this frigid gap save Lapras. Sadly, the meta has moved on, and nowadays Dewgong is nothing more than a dex filler. 

Best PvE Offensive Moveset

• Ice Shard* is Dewgong's best offensive Fast Move, but it's Elite, so Frost Breath is fine too. Blizzard is the Charged Move of choice.
• Liquidation would be a good option if Dewgong had a Water Type Fast Move, but that's not the case.
• Aurora Beam, Water Pulse, Aqua Jet, and Icy Wind all fall behind.

Best PvE Defensive Moveset

• Iron Tail has solid power and some counter-coverage against Rock Types.
• Ice Shard* is Dewgong's best Ice Type Fast Move option, but don't spend an Elite TM on it. Frost Breath falls behind, though.
• Liquidation, Icy Wind, and Drill Run all pull ahead on defense.
• Aurora Beam, Water Pulse, and Aqua Jet all fall behind on defense.

Ice Shard is Dewgong's best fast move due to its superior energy gains, however this is a legacy move. Frost Breath and Iron Tail both offer poor energy generation, though the former takes advantage of STAB to provide respectable damage output. 

The update to legacy Icy Wind following the December 2019 rebalance makes it mandatory for optimal use of Dewgong, partly due to its lesser energy cost, and partly due to its guaranteed debuff on the opponent. Starting in June 1 2023, Dewgong gained access to Drill Run, granting it near perfect coverage. Liquidation is a great move too, but Drill Run + Icy Wind is often all the coverage that Dewgong needs.

Using other charge moves like Water Pulse, Aurora Beam or Blizzard are direct downgrades due to their lack of utility and higher energy costs. If you don't have the double legacy moveset for Dewgong, maybe consider other Pokemon instead.