I had extremely bad luck with pubs in this game. Nine games out of ten I was running Lucio or Mercy - because no one else would - and desperately healing individual players who kept charging the entire enemy team all by themselves. The concept of hanging back for a bit and waiting until the rest of the team respawned and gathered up seemed to be completely beyond their understanding.
My advice - look around on gaming communities like Giant Bomb and see if they have a voice communication server running. I play with people from a GameFAQs spin-off site and it's so much more fun that way. I know Giant Bomb has a Discord server you can use, though to be honest I haven't seen anyone using it to coordinate an Overwatch team. It's totally doable, though. Add some battletags from the Giant Bomb team thread here and get cracking!