Yeah, don't bother with Hidden Powers... besides, I find you NEED all of Hydro Pump, Surf, Draco Meteor, and Dragon Pulse. You just don't perform as well overall without all those STAB options for power and consistency. There are so many turns when you simply can't afford to miss, and you need Surf or Dragon Pulse to be sure of a sweep and clinche the win against a rain team.
I personally find Kingdra is too weak and too frail using Life Orb, doubly so if you're taking turns to try and set up. Specs keeps it relevant regardless of match up, and lets you use Kingdra's turns of life to the fullest by attacking and attacking with those valuable turns. Life Orb Rain Dance does pull off sweeps that are otherwise impossible though, and can turn the whole weather war in your favor in an instant (getting swept by Venusaur? just rain dance and sweep the enemy team).
Usually though, Kingdra really wants room for all its special STAB moves to help it get its maximum utility.
In any case, it sounds like we're in agreement about Kingdra's C-Rank level threat :D
Edit:
Also Bronzong maybe also C level? (Moves in order of relevancy/importance: Stealth Rock, Hidden Power Ice (a must), Toxic, Gyro Ball, Earthquake)
Perfect counters: Mamoswine, Gliscor
Almost perfectly counters: Tornadus-T, Dragonite, Latios, Landorus-i, Landourus-T, Haxorus,
Is a fairly decent check/pivot against: Garchomp, Salamence, Latias, Gastrodon,
Are the really important ones. Of course there are a bunch of other threats it can help you control too.