Pros: Politoed is definetly the biggest advantage the water type has. It is the only non-ubers rain starter and can devestate so many teams on its own. IIRC water type has the most pokemon, when compared to other types. This allows for some diversity. You have options like Empoleon or Swampert for support, while also having pokemon like Kingdra, Gyarados, Cloyster, or Sharpedo for offensive pressure. Water-Type also has good neutral coverage and with Rain Support even unboosted water attacks can do a lot of damage to neutral resistance pokemon. Most Water-Type Pokemon also learn moves to help deal with their weaknesses, as most Water-Type pokemon learn Ice Beam to help handle Grass-Types.
Cons: Often once the rain dissapears water type pokemon have a lot more trouble when facing opposing weathers like sun or sandstorm. Mono water teams have difficulty dealing with alot of grass attacks especially if they are on a sun team, as then their own water stabs are weakened.
Notable Members: Politoed, Starmie, Tentacruel, Sharpedo, Gastrodon, Gyarados, Jellicent, Rotom-W, Azumarill, Slowbro, Cloyster, Kingdra(non SS) are all exemplary Water-Type Pokemon.