As for type coverage, it depends on what moves you give Kingdra or Garchomp.
Kingdra's usual set is the Sub + Dragon Dance set:
~ Dragon Dance
Which you'd only get the typing coverage by Water and Dragon (against Fire, Rock, Ground, and Dragon). Then again, you can also run a special set with Hidden Power Electric against opposing Water types, or Flying types.
Garchomp's SD set:
~ Swords Dance
~ Outrage / Dragon Claw
~ Fire Fang / Aqua Tail / Stone Edge
Can give you move coverage than Kingdra, depending on your preference (against Dragon, Fire, Rock, Electric, Grass, Bug, Flying, Ice).
-Fire Blast/Stone Edge/Superpower
EVs: 252 HP/4 Atk/252 SDef
Ability: Sand Stream
EVs: 252 HP/96 Atk/160 SDef