Water/Dragon would definitely suit Gyarados. Along with everything else that's been said, STAB Outrage on this thing would kill.
And just because it has "dragon" somewhere (dragonfly, for instance) doesn't mean it should be Dragon-type. Yanmega doesn't look the least bit like a dragon, and the only Dragon-type move it can learn is Twister.
There's also Ampharos, or "Denryuu" in Japanese. Why would you think that should be a dragon? The "ryuu" alone? The name itself is most likely from 電流 denryuu (electric current). It can also learn Outrage by tutors, but so can, like, a million other non-draconic Pokemon: Abomasnow, Aggron, Venusaur, Primeape, and many, many more.