Hey!! So first of all I would like to say that I have a mono water team of my own so hopefully that means I know what I'm talking about but I'll try not to portray my answer as the only right answer if you get what I mean. Since there's no such thing as a perfect team in Pokemon.
So first of all you seriously need a politoed on your team to get the rain up, I would maybe replace azumarill as I don't really think it's needed.
Politoed@Leftovers
Ability: Drizzle
EVs: 252 HP/252 SpD/4 SpA
Calm Nature
-Scald
-Toxic
-Protect
-Ice Beam
Now that you have rain up you can sweep using Swift Swim. Swampert should be max speed but you can keep it adamant. Replace dive with waterfall and hammer arm with protect so you get your boosted speed when you mega evolve. Alternatively you can replace protect with Stealth Rock
Your Kingdra set is nice and works well with your team, but I do have a potentially better set that works with the rain.
Kingdra@Choice Specs/Life Orb/Expert Belt
Ability: Swift Swim
252 SpA/252 Spe/4 HP
Modest Nature
-Surf/Hydro Pump/Scald
-Flash Cannon
-Dragon Pulse/Draco Meteor
-Signal Beam
Again, your rotom W set seems nice but you have trouble taking physical hits on your team. More just switching in to physical hits. A defensive set would probably work better.
Rotom-Wash@Leftovers
Ability: Levitate
252 HP/252 Def/4 SpA
Bold Nature
-Hydro Pump
-Volt Switch
-Will o Wisp
-Pain Split
Your Gyarados set is great. It's a needed Pokemon on a mono water team as it takes grass hits well. I also have one on my team but it's a defensive set that still hits hard. I could give it you if you want but you seem to have a nice strategy that works well already.
Vaporeon is also a great Pokemon. I have a specially defensive Tentacruel in this spot but this works well too. I think you should change the set slightly though.
Vaporeon@Leftovers
Ability: Water Absorb
252 HP/252 SpD/4 SpA or your current EV spread
Calm/Modest nature
-Scald
-Wish
-Acid Armor
-Ice Beam/Protect
I think that makes for a pretty good team. You have some great sweepers that utilise the rain and also some defensive pivots to balance the team out. I hope I helped. Let me know if there's anything you need :]
