To add to what Srinator said, Goodra makes a fine Assault Vest user, being able to switch to Electric moves should Swampert go down:
-Draco Meteor
-Fire Blast (still useful for hitting Ferrothorn and friends, despite it being weakened in rain)
-Thunder/Sludge Wave
-Earthquake/Muddy Water
Nature: Modest
EVs: 240 HP/252 SAtk/16 Spe
Item: Assault Vest
Ability: Sap Sipper
Wash Rotom should use a more defensive set if it goes bulky; otherwise, offensive variants should go fast:
-Volt Switch
-Hydro Pump
-Will-o-Wisp
-Pain Split/Rest
Nature: Bold/Calm
EVs: 252 HP/212 Def/44 Spe (Bold) or 252 HP/212 SDef/44 Spe (Calm)
Item: Leftovers/Chesto Berry
or
-Volt Switch
-Hydro Pump
-Thunder/Will-o-Wisp
-Trick
Nature: Timid/Modest
EVs: 4 Def/252 SAtk/252 Spe
Item: Choice Scarf
Mega Swampert should go over Ludicolo, and be considered over Mega Blastoise, as being stopped cold by special walls like Blissey sucks. Here's what it runs if you're interested:
-Waterfall
-Earthquake
-Ice Punch
-Low Kick/Superpower/Rain Dance
Nature: Adamant
EVs: 104 HP/252 Atk/152 Spe
Item: Swampertite
Here's the full set for Swift Swim Kingdra; while Srinator mentioned the moves, I feel like I should add the EV spread and preferred nature as well:
-Hydro Pump
-Draco Meteor
-Outrage/Rain Dance
-Waterfall/Surf
Nature: Rash/Modest
EVs: 64 Atk/252 SAtk/192 Spe
Item: Life Orb/Damp Rock
Ability: Swift Swim
Here's a full set for Tornadus, which is preferred as a backup rain inducer because it has Hurricane to pummel Grass Pokemon with:
-Rain Dance
-Hurricane
-Superpower/Focus Blast
-Knock Off/Taunt
Nature: Naive/Timid
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb/Damp Rock
Ability: Prankster