Well they did give us a Fire type that was resistant (actually did normal damage) to Water types, but that was a legendary (and as much as I enjoy Dragon types, I don't like Reshiram due to it's retarded cry) so it doesn't count. What I find funny is that while they don't give us a Fire/Water type, they instead let Rock types use the move Surf. Now I don't know what their reason is... but if it's something silly like "You can find rocks in the water so it's only logcial" then why can't they just let fire types use surf as well?
I'm probably tripping myself up big time here when I say that, but if Gamefreak can make all sorts of weird things, then they shouldn't have difficulty giving me a Fire/Water type. Never mind common sense, it doesn't exist in Pokémon (at least I'm not aware of it). GameFreak should be able to come up with something at least (knowing how they can come up with anything).