Shadow Swampert

Shadow Swampert is absolutely a Pokemon worth raising. It has some of the highest DPS among Water Types and viable Ground Type DPS as well. The one down-side is that you'll need to get rid of frustration via event and then evolve it to get Hydro Cannon during event hours or drop an Elite TM, so raising one without an Elite TM could potentially take a full year currently.

Note that Shadow Pokemon cannot mega evolve, but unless you find a Shadow with low stats that will purify to something worth using, it's best to use it as a Shadow.

Best PvE Offensive Moveset

• Water Gun should be paired with Hydro Cannon. This set is better than the Ground-type set against most targets, including Rock-types.
• Mud Shot should be paired with Earthquake. This set has an advantage against Steel, Electric, and Poison-types.
• Sludge Wave is underwhelming in every aspect.
• Surf has been replaced by Swampert's Community Day move. 
• Muddy Water as a 3-bar move can be used in a pinch right before Swampert faints, but otherwise is inferior to even Surf. 

Best PvE Defensive Moveset

• Mud Shot is slightly superior to Water Gun, offering slightly better coverage and cross-weather benefits.
• Hydro Cannon is superior to Sludge Wave and Surf, having a lower energy cost than the former and earlier dodge window than both.
• Muddy Water can be used more often as a 3-bar move, though it has a later dodge window than Hydro Cannon. 
• Sludge Wave offers counter coverage against Grass-types, however.
• Earthquake is underwhelming in every aspect despite having STAB.

Mud Shot offers the highest energy generation in the game at the cost of low damage output. Due to how powerful Swampert's charge moves are, this move is recommended. Water Gun offers more damage output and is meant more for general coverage, but is not recommended due to its inferior energy generation. 

Hydro Cannon is a move limited to specific events such as Community Day, as well as Elite TMs, and is mandatory due to its lesser energy cost and very high Damage Per Energy value. It also completely blows Surf out of the water. Earthquake and Sludge Wave are both viable options, with Earthquake giving more power against Steel-types and Sludge Wave letting you hit Fairy-types harder.

What about Muddy Water? 

Muddy Water is a very disappointing addition to Swampert for PvP. Hydro Cannon deals roughly 257% of the damage that Muddy Water can do, for only extra 5 energy. While Muddy Water does have a perk of reducing the opponent's ATK stage by 1, the 30% chance to proc the debuff makes it unreliable to use for consistent results.