Your team fails to use the strengths that Rain as a playstyle has: Extremely powerful, extremely fast hyper offense. You have one swift swim sweeper, and Manaphy, as the only mons who truly abuse the Rain that Politoed provides. Right now, it's just a balance team that's trying way too hard to abuse Rain, only failing to do so because it's so slow.
Your Politoed set is extremely bad. It doesn't hit hard enough without a boosting item to warrant two STAB moves, especially Hydro Pump, and Psychic doesn't even beat the Pokemon it's meant to hit, mainly Plasmanta and Mega Venusaur:
8 SpA Politoed Psychic vs. 4 HP / 0 SpD Plasmanta: 124-146 (47.3 - 55.7%) -- 75.8% chance to 2HKO
8 SpA Politoed Psychic vs. 232 HP / 4 SpD Mega Venusaur: 102-122 (28.4 - 33.9%) -- 0.7% chance to 3HKO
You don't even 3HKO a mon who's weak to it; that's how pitifully weak it is. Also, you're running Protect. You're burning and stalling out your own rain turns. Jirachi also burns about 2-3 turns of rain when it comes in, so it is actively hindering you. Rotom doesn't have the strength to abuse Hydro Pump with that set, and it lacks Thunder too, which is another bad moveset.
You're running Outrage Swampert. You're set up bait for every Fairy and Steel with that move, and it's just atrociously bad on a mon who doesn't get STAB on it.
Your team fails to do what Rain does, and that's breaking apart teams quickly and efficiently. You don't use Kingdra, Plasmanta, Omastar, Kabutops, Krilowatt, Keldeo, Tomohawk, Tornadus-T, and many, many other Pokemon who abuse the rain FAR more effectively, and without wasting too many of your precious rain turns.
Now, instead, I'm going to actually propose the exact opposite and propose Politoed to B+, for the following reasons:
Rain as a playstyle in general is very underprepared for, and VERY powerful in CAP with so few Tyranitar and Charizard-Y running around. Balance crumbles to it, and the rise of Offensive teams bodes well for it, as Rain can outspeed and pummel them with Life Orb Hydro Pumps and Waterfalls. Politoed is an absolute necessity for Rain, and thus, should raise to reflect the amount of power it has.
Swampert is a rain staple because of its immunity to Electric and the Earthquake coverage that pairs well with Waterfall, hitting things like Mollux with otherwise is quite a problem for Rain teams. While it lacks the item that makes Kingdra and Kabutops hit so hard, it has a hefty 150 Attack stat, the ability to run Adamant comfortably, and Rain + STAB boosted Waterfall, which hits very hard, and is very fast. It has the drawback of Mega Evolving to abuse Swift Swim, but that's minor compared to the large amounts of benefits it provides rain teams.
Sadly, I cannot provide replays, I can only use so many different teams and Rain isn't one of them for now. I'll post replays to further back this up when I get the chance.