Hey Makaha...
I have used gas powered hookah units for shallow underwater work (hull cleaning, mooring repairs, zincs, props, etc) and have accumulated thousands of hours on them. I have had to two unfavorable episodes (accidents, oh-crap, now what? situations) with them.
The first one was while cleaning a commercial fishing boat at the dock. I brought the compressor down to the dock side, was halfway through cleaning the hull and all of a sudden, the compressor stopped. Just prior to the unexpected stop, the hose filled with water and a cup full was pumped into the mask...

The compressor "walked" off the edge of the dock after a couple of good sized boat wakes.

Lesson learned... tie down/secure the compressor to the dock somehow or to the dock cleats.
The second one was during hull cleaning again, this time the compressor was mounted in my dive boat. and all of a sudden, I felt nauseous, dizzy and light headed. I surfaced and threw off the mask (full face commercial EXO-26) and barely made it back into the boat. I found that the bolt holding the "snorkel" breathing air intake filter, had snapped and the intake filter fell over and landed about two inches from the gas engine exhaust muffler!!!

Lesson learned... check the mounting hardware of the breathing air intake assembly or remote the breathing air intake assembly higher and farther away from the gas exhaust muffler.
In both situations, I was at about 7 feet of water and was able to surface safely and quickly on half a breath of air. Had I been at a much deeper depth, the outcome may not have been very positive. The second episode left me with a severe headache and nausea from the carbon monoxide poisoning... but I survived and was happy to be back at work the next day.
The equipment is essentially safe, you just have to keep up with the maintenance, keep the fuel tank filled, anticipate mishaps and take appropriate precautions. Good luck... hope this helped,
Wayne