Main things you need to worry about are Electric and Grass liabilities. Currently, all your Pokemon are weak to Electric, Gyarados having a x4 weakness (which means a non-STAB Thunderbolt will OHKO it), and only Gyarados is neutral to grass.
Since you have Politoed on your team, this team will be OU. Bibarel, Lapras and Carracosta are not suited to OU (Lapras is a maybe with Hydration, but if you were going to use this ability in the rain then really it should have Rest). I also wouldn't have Carracosta and Cloyster on the same team; I would have one Shell Smasher and another Choiced Pokemon. Shell Smash leaves you open to too many things, especially without White Herb. One grass move will OHKO Carracosta and one special Grass/ Electric move will OHKO Cloyster.
I'm suggesting other Pokemon in replacement of these 3, in order to cover your Grass/ Electric weakness and to allow this team to function better in OU.
Politoed @ Leftovers
Ability: Drizzle
EVs: 252 Def, 252 HP, 4 Sp. Atk
Nature: Bold
Scald
Toxic
Perish Song
Protect
Defensive Politoed, since your Pokemon will depend on its Drizzle. Perish Song is there to get rid of Calm Mind/ Bulk Up stallers, and Politoed can wall a few hits with its bulk + Protect.
Rotom-Wash @ Leftovers
Ability: Levitate
EVs: 232 Sp. Def, 200 HP, 76 Sp. Atk
Nature: Calm
Will-O-Wisp
Hydro Pump
Volt Switch
Pain Split
Rotom-Wash will cover your Electric weakness. Will-O-Wisp is there to deal with physical sweepers, meaning Rotom-Wash can take a hit or two.
Vaporeon @ Leftovers
Ability: Hydration
EVs: 252 Def, 252 Sp. Def, 4 HP
Nature: Bold
Rest
Heal Bell
Scald
Ice Beam
Rain stall Vaporeon. You can choose Wish as part of its moveset for more team support, but imo this current set works just fine on a rain team.
Keldeo @ Choice Scarf
Ability: Justified
EVs: 252 Speed, 252 Sp. Atk, 4 Hp
Nature: Modest
Secret Sword
Hydro Pump
Surf
HP Ice
You can consider a Calm Mind, Life Orb or Choice Specs set in favour of Choice Scarf, especially to get rid of Pokemon like Blissey, but Choice Scarf works just fine from personal experience. Even if you did need 2 Secret Swords to get rid of a Blissey, it's not like it can do that much damage to you.
Cloyster @ White Herb
Ability: Skill Link
EVs: 252 Atk, 252 Speed, 4 Sp. Atk
Nature: Naive
Shell Smash
Icicle Spear
Rock Blast
Hydro Pump
You can give it Focus Sash in favour of White Herb, but having 1 HP left will make it susceptible to priority moves. Standard set for Cloyster; you don't need too much investment in special attack because Hydro Pump will be boosted by the rain.
Tentacruel @ Black Sludge
Ability: Rain Dish
EVs: 252 HP, 252 Def, 4 Sp. Atk
Nature: Bold
Rapid Spin
Scald
Protect/ Ice Beam
Toxic Spikes/ Knock Off
Tentacruel will cover your grass weakness. It will also soak up Toxic Spikes and get rid of other entry hazards; main thing it's there for is team support.
Other options
Quagsire @ Leftovers
Ability: Unaware
EVs: 252 HP, 252 SP. Def, 4 Def
Nature: Caerful
Curse
Recover
Earthquake
Waterfall
Bulky sweeper. Unaware is really brilliant, especially against Dragon Dance Pokemon. Quagsire will be slow anyway, so boosted speed makes no difference. Watch out for Grass moves and status moves though.
Gyarados @ Leftovers
Ability: Intimidate
EVs: 180 HP, 132 Atk, 204 Speed
Nature: Adamant
Dragon Dance
Waterfall
Stone Edge
Earthquake/ Taunt
I'm not much of a fan of Gyarados; I've used it before and I've always found that while it'll deal a lot of damage, it's too frail to keep going. It's OHKO'd by a non-STAB Thunderbolt, so it's not as if it's hard for your opponent to get rid of. Still, this set gives it some bulk so it can set up before getting swept.
Kingdra @ Life Orb/ Lum Berry
Ability: Sniper
EVs: 72 Atk, 200 Sp. Atk, 236 Speed
Nature: Naughty
Dragon Dance
Waterfall
Outrage
Ice Beam/ Draco Meteor
Due to brilliant typing, Kingdra will be neutral to both of your main threats. No Swift Swim though as this is OU, but if you're taking it to Ubers, then use Swift Swim as its ability.
