You don’t “make oxygen from water.”
A molecule of water is an oxygen atom bonded to two hydrogen atoms.
You can break the bonds of two water molecules and get an O2 molecule and 2 H2 molecules, but the oxygen already existed and was not made, which would require a nuclear reaction.
A Star Ship would have unlimited nuclear power.
Just a hint, take 0.0000000000001% of that energy and split water. However, you will not win research grants.