Who, besites me, wants a Water/Fire Pokémon?
It could have a decent HP with good bulk, a good Special Attack, and an excellent speed. With Water Absorb, this Pokémon would be able to wall both sun and rain teams by being 4x resistant to Fire and immune to Water. With Air Lock and a good speed, it could be an excellent revenge killer of weather sweepers. It would also be the only Water-type that wouldn't fear Ferrothorn at all, as it would OHKO it with a Fire-type attack. If this Pokémon were to exist, I hope that it got Thunderbolt as a coverage move to cover bulky Water-types that would resist both of its STABs.
Water/Fire is an excellent typing, both offensively and defensively. It brings three 4x resistances: to Fire, Steel, and Ice! as well as a resistance to Bug. It would be weak to Stealth Rock, unfortunately, but this is nothing that Rapid Spin support can't solve. It is, unlike other bulky Water-types, neutral to Grass-type attacks. Being part Fire-type would probably mean that it would not get an Ice-type attack like Ice Beam. This is not that bad, as this Pokémon could still cover Grass-types with its Fire-type STAB; but it will have a harder time against Dragon-types, having to resort to Hidden Power Ice to overcome them. However, this is a fair price to pay for the fantastic typing.
Also, the above user should reserve that reaction for the moment that Celebi would get Quiver Dance. CELEBI NEEDS THIS MOVE ON ITS MOVESET! It would sweep better than the Nasty Plot set. I could see QD Celebi being the standard set. The problem is that Celebi will have a harder time OHKOing Dragon-types without Hidden Power Ice, especially Dragonite, as Quiver Dance does not double the power like Nasty Plot does. However, being able to boost its Special Defense and Speed at same time is a fair price to pay for this.