Type: Water/Ground
Classification: Mud Fish Pokémon
Height: 6'03" / 1.9m
Weight: 102.0kg.
Ability: Swift Swim
After Mega Evolving, all of Swampert's stats increase, where its Attack stat getting the highest boost and its Defense and SP Def stats following suit. It doesn't suffer with any stat decrease after Mega Evolving which definitely makes it a more powerful Pokemon after transformation. Its Swift Swim ability also doubles its Speed stat in the rain so Mega Evolving while there's an ongoing rain will ensure that you'll have a fast, powerful hitter with well-rounded stats.
Reference and images: Serebii.net
Additional references, images, and information: pokemonrubysapphire.com