I mostly shoot photos, very few films as I don't have the patience to edit clips! (a few on my instagram)
If i do film, I start filming once at depth to be honest
My "workflow" is :
Turn the camera on and get appropriate settings
breath up (no hyperventilation)
Duck dive while pulling the camera behind you (rather than pushing it forward)
position yourself vertically at the desired depth (don't be too harsh on the turn, you don't want a squeeze if you're at 30-40m depths)
Start filming
Go up (by being vertical, you're already in position to go up)
If the subject is moving (dolphins, whale shark), then really it'all about keeping up and filming.
Don't be surprised if contractions come quicker than in the pool as you will be much more active with a camera in the hand (more Co2).
Not sure i can help more though, let me know