There's been asked a Kingdra question just recently here. Due to Kingdra's double resistance to water you want STAB dragon moves to counter Vaporeon.
I don't see where a Kingdra with water moveset would have an advantage over other strong water types (like Vaporeon)
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I agree. There are very few mons with a double dragon STAB moveset. That is what you want.
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My Kingdra has WG/Outrage. Is it any good?
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Mine has the same, I guess that's the best MS (even according to PokéGenie)