Poseidon Mk IV Discovery Rebreather
You can't quite breath underwater forever, but, longer than you normally could. The Poseidon Mk IV Discovery is what those in the diving world call a rebreather - it recycles a user's oxygen. Using p02 scrubbing and oxygen cells, it turns unused oxygen in your exhales into breathable air. Supposedly, the average person only uses about 25% of the available oxygen in the air. The exhale contains the other 75% of oxygen along with carbon dioxide and nitrogen. It's smaller, weighs less, and even costs less than traditional diving tank systems.