A good technique might be for the DM to ask the diver(s) if he/she/they want help at all.
For example:
"Hey Joe, are you happy with your gas consumption? (...or, finning technique, trim, buoyancy control, streamlining, dive planning, hand signaling, etc.)
It the diver says, "yes", then that's it. If the divers says, "no", then the DM is being invited to teach.
And what about a DM that could use some improvement? Do you think the DM would be happy to be instructed by a diver without being asked?
I think not.