1. Hindsight is often 20/20. That said, it's important to learn from mistakes and risks.
2. These days, I always carry 2x line-cutters and position them where I can easily find/use them "blind." Knives I usually avoid due to size and risk of stabbing yourself.
3. After a minor dive-incident I had (below), I now regularly practice being able to retrieve all equipment "blind." For example, my spare regulator and cutting devices are positioned on my BCD harness, precisely where my opposite hand naturally reaches across my chest.
As a side-note, I prefer tools like these over knives:
The Sharp Cut by Sea Sword has a very sharp double-sided CERAMIC blade that provides an incredibly effective cut.
www.divegearexpress.com
The DGX M-Cut is a lightweight and corrosion resistant cutting tool.
www.divegearexpress.com
Specifically, knives are a pain to sheathe, and there's increased risk of stabbing yourself.
I also strongly recommend 2x cutting devices. If one is lost, or an arm is tangled, ideally position them where you can easily reach "bindly" by feel and instinct. Mine are on my harness, on my chest.
Point your thumbs up, and them point your thumb to your chest. It should be about between your pecs and shoulder. I've found this to be an ideal position for important equipment; you can reach it from either hand and it's very consistent. This is the precise place where my extra regulator is (right-chest) and cutting devices (both sides).
The suggestion of mounting on wrist-computer(s) is also a good one.