When I defeated the 7th Gym in alpha Sapphire, there was a pillar of light coming from the Seafloor Cavern. I then talked to Steven and he gave me the HM Dive and told me to go and stop Team Magma and Team Aqua, but I can't find them! Does anyone have any tips as to where to find them because I'm really stuck! (I have been to the Seafloor Cavern)
Once you dive down to Seafloor Cavern you need to resurface. If you are in the right place, you will find the submarine that Team Aqua stole, When you see it go to land and walk inside the cave. You will find grunts that you need to battle. Get through them and you will find Archie talking to Maxie. Archie will then ask you to battle him. (This is all based on Alpha Sapphire but you do the same thing in both games.)
You'll actually want to travel to Seafloor Cavern to find Team Magma or Aqua, which can be found on Route 128.
It should look similar to this: