Kyogre's DPS puts Gyarados's to shame, so you probably don't need another good Gyarados, given the water Mons you listed. That said, Karps seem to be spawning like crazy during this event, so if you're willing to be patient, I'd say the odds are good that you'll come across a good one.
Otherwise, if you'd prefer to move your special research along sooner rather than later, personally, I'd evolve the shiny. Just because red Gyarados looks amazing.