Gyarados= Gyser + ''Radeau'' (French for ''Oar'', thus relating to the Oarfish, which shares Gyarados' long, snake-like body) + Adolescent
With the Oarfish relation, and the Gyser and Adolescent signifying its explosive temperment
Another theory:
Gyarados= Gyro (since it's a Flying-type as well) + Radical
Meh...just guessin' here.