Kingdra Description

Kingdra, The Dragon Pokémon. 

It lives in caves on the seafloor and creates giant whirlpools every time it moves.It is said that it usually hides in underwater caves. It can create whirlpools by yawning.
Overview

Kingdra is a well rounded Pokemon stat-wise with an excellent typing as well as a great mixed movepool, with good abilities to boot. The only reason Kingdra isn't solidly OverUsed is because its stats, though quite good, are just a little subpar, especially in its Speed compared to many of the other Dragons in OU. The ban on Drizzle + Swift Swim by most competitive sites also did not help Kingdra any (although since this is still legal in Doubles, to my knowledge, Kingdra is very effective there). However, Kingdra should never be discounted as in UnderUsed it can provide its own Rain to sweep with, or run a different but just as viable set, while in OU it makes possibly the best counter to Rain a team can have.

Abilities
Swift Swim: Kingdra's Speed is doubled under Rain, an excellent ability although banned with Drizzle on the same team in most places, however it can still be used with Drizzle in Doubles as well as with Rain Dance (or of course if you aren't running Rain). Easily the best of Kingdra's abilities.
Sniper: Boost the power of critical hits, although not a great ability it is still probably the one you want to run if you aren't running / can't run Swift Swim.
Damp: Prevents the opponent from using Selfdestruct or Explosion, not that useful of an ability anymore as Explosion was massively nerfed between 4th and 5th gen, although if you run into a shiny Graveler, this is the ability you want.
Movesets
I Could Eat A Seahorse
-Substitute / Rain Dance / Draco Meteor / Rest
-Dragon Dance
-Waterfall
-Outrage
Item Attached: Lum Berry / Leftovers / Life Orb / Chesto Berry
Ability: Swift Swim / Sniper
EVs and Nature:
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature (+Atk, -SAtk) or Lonely Nature (+Atk, -Def)
This set can be a little convoluted with a few of the choices, but I'll explain the best I can. Dragon Dance is a staple on this set as it provides Kingdra both with more Attack and more Speed, both of which it can use very much to its advantage. Waterfall can do quite a bit of damage against quite a few Pokemon (especially Steels not named Ferrothorn), and under Rain becomes an even more powerful STAB move. Outrage is obscenely powerful and also gets a STAB boost, and this coupled with some Dragon Dance boosts can lead to you decimating your opponent's team. The last option (well, the first slot) is really dependent on what sort of set you want. Substitute is commonly seen on Kingdra as it provides a shield from status effects as well as giving Kingdra a buffer to get a Dragon Dance up. Rain Dance provides Kingdra with a significant Speed boost (very effective in UU where there is no other source of Rain), allowing Kingdra to often do a lot of damage to a team without even needing a Dragon Dance boost. Draco Meteor makes it so Kingdra can power through some Defensive walls that might otherwise give Kingdra trouble (also note that this is the sole reason that Lonely Nature is an option, so use that with this if you choose). And finally Rest gives Kingdra a good source of recovery, and when combined with Chesto Berry leaves Kingdra with no repercussions. Moving onto items, as I said previously, Chesto Berry is only useful with the Rest set, so only use it if you're running Rest. Lum Berry helps to avoid status as well as cure Confusion that inevitably comes with the use of Outrage. Leftovers provides a steady source of recovery, allowing Kingdra to survive longer, set up more Substitutes, and set up more boosts as well. Life Orb can be used if you want Kingdra to have more initial power (useful on the Rain Dance set as you may not want to waste a turn using Dragon Dance) although the recoil will wear down Kingdra fast.
Just Keep Swimming
-Hydro Pump
-Draco Meteor
-Dragon Pulse
-Surf / Hidden Power [Electric] / Rain Dance
Item Attached: Choice Specs / Life Orb
Ability: Swift Swim / Sniper
EVs and Nature:
EVs: 4 HP / 252 SAtk / 252 Spe
Modest Nature (+SAtk, -Atk) or Timid Nature (+Spe, -Atk)
This set focuses entirely on the Special Attacking side of Kingdra, and uses its many tools to become quite a dangerous threat. Hydro Pump provides powerful Special STAB, though the accuracy always seems to kick in when you don't want it to. Draco Meteor is the be-all-end-all move for Kingdra, as it often has no choice but to switch out afterwards due to the drop in Special Attack that follows its use. Dragon Pulse is the less risky version of Draco Meteor, but with a lot less power behind it. The last slot depends on what item you're running. If you're running Choice Specs, then you are obviously not going to be picking Rain Dance. Surf is to Hydro Pump as Dragon Pulse is to Draco Meteor, more reliable but weaker. HP Electric helps take care of bulkier Water types in a case where they resist Draco Meteor (such as Empoleon). Rain Dance should be run with Life Orb and provides a crucial Speed boost. It is also worth mentioning that this set, either Choice Specs or Life Orb, is extremely useful in Doubles, as it has very potent wall breaking capabilities, as well as being aided by the allowance of Drizzle and Swift Swim on the same team. The choice between natures is up to you, whether you want more power or more speed.
Other Options
Bounce, Muddy Water, Protect, Ice Beam, Blizzard, Scald
Blizzard does the same as Ice Beam, but it offers more power in exchange for less accuracy.
Bounce is a two turn move that offers coverage against Grass types, although lack of a STAB boost and the fact that they'll know exactly what you are doing won't help out Kingdra much.
Ice Beam does the same things for Kingdra as Blizzard, but with more accuracy and less power
Dive is a two turn Water type move that offers a physical STAB option, but Waterfall is infinitely superior and, like Bounce, gives too much of an indication of what you're doing.
Muddy Water is an alternative option to Surf. It gives a chance for a decrease in accuracy in exchange for having lower power and lower accuracy itself, so generally not advisable.
Protect is a great scouting tool that allows Kingdra to see what its opponent is planning, especially useful in Doubles.
Scald is a good special STAB option with a bonus chance to burn its opponent, very useful for crippling physical attackers as well as doing decent damage under Rain.
Double & Triple Battle Options
Kingdra works really well in conjunction with Politoed and Drizzle, the advantage lying with Kingdra’s ability Swift Swim. Tack on a Choice Specs, and put EV investment in Special Attack and Kingdra becomes a special sweeper that will destroy everything in its path. Boosted by the Rain, STAB and Choice Specs, Hydro Pump can OHKO many Pokemon that do not resist it, and severely damage even those that do. Swift Swim Kingdra even has the advantage of outrunning several Choice Scarf Pokemon with the Swift Swim boost, so it is definitely a good option for having in Doubles and Triples.
Partners
If you're in UU, Rain Dance support for Kingdra is much appreciated, as well as anything that can rid it of Empoleon, which otherwise blocks Kingdra at every turn. In OU, Kingdra really just needs something to eliminate Ferrothorn, which is a bigger thorn in its side (ba dum tiss) than Empoleon. In Ubers and Doubles, Drizzle support is extremely useful, and basically all that Kingdra needs to sweep.
Countering Kingdra
I'll just cover the UU counters because otherwise you'll be reading this until X and Y come out. Anyhow, Empoleon is easily the best counter to Kingdra, resisting both of its STABs while also being able to phaze it out. Another decent counter is the rarely seen Ferroseed which serves the same purpose, but can set up hazards instead of phazing out. Attacks from faster Pokemon can often hurt Kingdra's sweeping powers, with Scarf Pokemon like Flygon and Darmanitan being able to do serious damage if not KOing. Bulky Pokemon on both sides of the spectrum can handle Kingdra, depending on its set, so Pokemon like Cofagrigus, Porygon2, Milotic, and Snorlax all can generally wall Kingdra. Another good check (although not a Pokemon) is residual damage through Toxic Spikes, as it limits how long Kingdra can stay in or what it does (won't want to Substitute with Toxic draining away its health), so Toxic Spikes setters like Qwilfish are an excellent check.

Locations in Games
Ruby/Sapphire/Emerald:
Evolve Seadra
FireRed/LeafGreen:
Evolve Seadra
Colosseum/XD:
Trade from Ruby/Sapphire/Emerald/FireRed/LeafGreen
Diamond/Pearl/Platinum:
Evolve Seadra
HeartGold/SoulSilver:
Evolve Seadra
Black/White:
Routes 17 & 18, P2 Laboratory
Black 2/White 2:
Routes 17 & 18, P2 Laboratory