FWIW...
I wear tethers on my fins, so putting them on in waist deep water is a pain.
I put them on while still on the beach. Since I wear Churchill's, I can pretty easily duck-walk to some point in the water, in which I'll turn either completely backwards or side-step... depending on currents, visibility, holes in sandbars, etc.
If it's a rock/reef bottom, I'll have my leash pulled taut from the ankle and held by the hand with the board. This will prevent looping around stuff on your way out, including your finned feet.
The remainder of the leash length has some give in case I need the slack. I'm always ready to let the taut part of the leash go if I need to.
Then there's jumping off of rocks... to be discussed later...