You can’t breathe pure oxygen any deeper that 6 meters (less than 20 feet). Air is 21% oxygen and 78% nitrogen (with other gases making up 1%). Splitting water makes 100% oxygen and you have to remove the hydrogen because of. explosion hazard.
“Breathing air containing 21% oxygen risks acute oxygen toxicity at depths greater than 66 m; breathing 100% oxygen there is a risk of convulsion at only 6 m.”. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1114047/