I've played some Mix and Mega... I don't think Swampert is necessarily Bad, but, as sumwun said, it is outclassed by Golisopod. Also, the ability Torrent does nothing as you will Mega Evolve; while, Damp may save you from an occasional Explosion. Damp is therefore more useful on Swampert by default. Still, I don't believe it is unusable. Here are Mega Stones I recommend:
Blue Orb: Primordial Sea, 100 HP, 160 Atk, 90 Def, 115 SpA, 110 Spd, 60 Spe This will make Swampert's Waterfall hit ridiculously hard, and make Swampert into more of a Special Tank. :P
Metagrossite: Tough Claws, 100 HP, 120 Atk, 110 Def, 95 SpA, 110 SpD, 100 Spe Tough Claws Boosts all of Swampert's moves except Earthquake. This set has the Lowest Attack (but Attack is fixed with Power-Up Punch and Tough Claws), but is the bulkiest and the fastest.
Lopunnite: Scrappy, +Fighting, 100 HP, 170 Atk, 100 Def, 85 SpA, 90 SpD, 90 Spe This set makes Power-Up Punch more spammable and gets rid of that x4 Weakness (if your worried about that sort of thing). This set can hit hard, but lacks some bulk in comparison. :P
Lucarionite: Adaptability,100 HP, 145 Atk, 108 Def, 110 SpA, 90 SpD 82 Spe Swampert's Extra attack and Speed, as well as Adaptability making Earthquake and Waterfall hit devastatingly hard.
I'd say Metagrossite and Blue Orb are your absolute best options, unless you alter you set to fit the stone better. :P
You also might want to use the /mnm pokémonName@stoneName command in Showdown! to help you choose team members. :P
Source: EXP
Hope I Helped!
