Breath underwater indefinitely by splitting Carbon dioxide in the lungs

Lasers can split the CO2 in the lungs into C and O2.  If the CO2-splitter disposes of the Carbon safely, the O2 would be reused by the erythrocytes' hemoglobin indefinitely.

With proper safety gear for ears, mouth, and eyes, one might "scuba dive" to 500 or more feet indefinitely (until you became hungry).

Some initial research on splitting CO2 using UV-light:

http://phys.org/news/2014-10-oxygen-molecules-carbon-dioxide.html