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It's really difficult to breed 6IV Pokemon. I usually just breed for 5IVs because typically that's all you need, really. One of the attack stats of a Pokemon is usually useless.

Anyway, if you're looking to put together a mono-water type team then the obvious weaknesses your team will have are electric and grass.

Swampert - Great choice. It's ground typing nullifies electric attacks, but grass attacks will rip it apart with ease. Beware of Contrary Serperior.
Nature: Something that increases either defense and drops special attack or Adamant.

I usually run Waterfall, Earthquake, Avalanche and Curse on Swampert. If you can Baton Pass to it, even better. But typically it's a bulky, hard hitting Pokemon. Maybe Rest might help it out if you're willing to lose the Mega Stone for a Lum or Chesto Berry.

Mega Sharpedo - Speed Boost is amazing. Use Protect to get the first +1 to Speed without any worries and then follow up with some hard hitting attacks. Adamant nature with the standard 252/252 physical sweeper EV spread should be adequate. Think: Ice Fang, Waterfall, Crunch, Earthquake, Aqua Jet (priority), etc.
Nature: Adamant or Jolly

Not too sure about Lanturn..

Kingdra - Great typing. Few weaknesses. Plenty of resistances. Not weak to Water or Grass. Maybe make it a bulky Dragon Dance user. Or perhaps go full special with Choice Specs? This is such a diverse Pokemon with plenty of options. Experiment and look online for other people's ideas. But it can definitely be worthwhile on a team. Maybe even make a stall/annoyance type of Pokemon out of it with Toxic and Protect.

Seaking - There are plenty of Pokemon that do what this thing will potentially do for you much better than it will be able to. Seaking with a Choice Band might be a thing..other than that its low stats are really a detriment.

Instead consider a bulky Gyarados. Good special defense and it isn't weak against grass type attacks, but it's dangerously weak against electric attacks so be aware of Volt Switches. Taunt and Thunder Wave are great moves to annoy enemies with. Slap him with Leftovers or Sitrus Berry for some decent recovery options.

Out of personal preference I'd run Milotic over Walrein. I have my very own Milotic that has proven to be pretty useful in Battle Spot Matches. It has reasonable bulk and plenty of HP to soak up both physical and special moves with the right EV spread. Ice Beam and Scald are necessary moves for coverage as they'll keep Fire, Rock, Ground, Dragon, Flying and Grass types under control. The 30% chance to burn with Scald can ruin opposing physical attackers. Couple that with moves like Recover, Attract, Confuse Ray, Toxic, Protect, etc. and you can have a really annoying AND damaging Pokemon in Milotic. I believe the ability mine runs is Competitive. They switch in something that has Intimidate and Milotic becomes a monster by getting +2 Special Attack.
Nature: Bold or Modest

Consider:
Starmie - High Special Attack, High Speed. With Choice Specs or Life Orb, it can be a great special sweeper. Learns moves like: Thunderbolt, Surf, Ice Beam, Psychic, Shadow Ball, etc. Great coverage!
Nature: Timid or Modest

Greninja - Protean is a must have Hidden Ability. This thing is probably the best damn Water-type special sweeper in the games. Go Timid. Moves: Grass Knot, Surf, Extrasensory and Ice Beam. Hold a Life Orb.
You'll be getting Super Effective/STAB hits against: Fire, Rock, Ground, Fighting, Poison, Flying, Dragon, Water and Grass types.