Why cant we invent a contraption that extracts oxygen from water to breath underwater?

the chemical formula for water is H2O, this means that two hydrogen atoms are chemically covalently bonded to a single oxygen atom. to get the oxygen the hydrogen has to be taken off, this requires a lot of energy to break the bonds. while it is possible for instance some deep sea vessels do it so they dont need to resurface, to do it for a personal use would not be feasible.