I'll try my best to fix this team up.
• Politoed is nice but switch water gem for leftovers since it nice to keep that fellas healthy, and change Hydro Pump for Scald for better accuracy and a chance to get a burn.
•Poliwrath won't do well in out switch him out for this guy
Gyarados- Choice Scarf
Evs- 4 Hp/ 252 Atk./ 252 Speed/
Works well under rain and it's scarf so this guy is a threat.
•Change out Swampert due to grass weakness and replace him for Ferrothorn.
Ferrothorn- Leftovers/ Rocky Helmet
Trait- Iron Barbs
Evs- 252 Hp/ 252 Def./ 4 SpD/
~Power Whip/ Leech Seed/ Gyro Ball
Works well under rain as it take advantage of fire type pokes and with those max out def and sp.defense.
Heatran is nice but fire gem is kinda useless under rain change that and give him leftovers or air balloon- (i rather go for air balloon ) also change Fire Blast for HpIce or HpGrass since you already have Lava Plume and nature to Modest or Timid.
Slowbro is nice but Tentacruel is always open as he's an rapid spinner and supports Gyarados as he's weak to rock obviously.
Blissey is alright.