How to create a safe DIY Snuba Apparatus Answered
My Idea so far is to use an air pump (maybe one similar to the one that pumps up inflatables?) is to feed air down a tube to a modified snorkle mask with a 1-way valve going in, and a one-way valve feeding just into plain water. The air pump would have filters and a catalytic carbon-monoxide filter/converter (w/ small heating element) in the tube leading down to the mask.
The idea is that there is always a fresh supply of air running through the mask that there could be no deadly build-up of bad gases within the small area. Included is a picture of how I envison it (minus the lame Microsoft Paint drawing skills).