This is a technical area. Please stay on-topic and be civil and respectful to others. We are all here to help birds and wildlife remain healthy and alive, even if our opinions may differ.
The standard Mirage fins hang down approximately 13" below the bottom of the boat and the Turbo fins hang down about 15". And keep in mind that the bottom of the boat sits a few inches below the top surface of the water. So in order to use even the standard fins you will probably need at least 17"-18" of water, maybe more. Add two more inches for the Turbo fins. And you can be in 100 foot deep water but if you glide over a 99 foot tall rock, you will hit it. So it is helpful to know your lake or river.
If you hit an object with a moving boat weighing hundreds of pounds you will at the very least bend the steel shaft that runs through the leading fin. You can also damage the lower section of the drive or even crack the hull. The fins can be pulled up tight against the bottom of the boat by holding one pedal all the way forward, or using the bungee to lock the right pedal all the way back. That's fine for grass, but rocks or solid stumps that rub against the hull can still damage the fins and a strike of the lower unit of the drive against a rock could do some much more expensive damage. So it is best not to hit anything.
What we do.
We use the drives to give us range. We paddle out from the launch beach until the water is deep enough and then drop the drives into their sockets. Sometimes we may pedal for up to two or three miles before we reach our work area for the day - an easy thing when using your legs for power. When we approach shallow water or the shoreline to do our work, we pull the drives out and paddle. The paddles give us excellent low speed maneuverability without the risk of expensive damage.
The Mirage drives are large and bulky. When they are pulled there is really no great place to store them. They lay athwart the sides of the boat and occasionally have to be grabbed away from branches that try to pull them overboard. A tether is highly recommended.
If anyone knows of a better way to stow the drives in the boat while we work, I would love to hear about it. They do need to remain accessible as we pull them and drop them off and on during the day.