Let us say hypothetically they could split water into Hydrogen and Oxygen and then create power through Hydrogen fusion reactions turning Hydrogen into Helium

And you gained more power then you put in

I have heard people argue this would be great because there is so much water on earth

It still might not be a good idea because you would deplete the earth of water and might be environmentally worse than just using the uranium that already has been gathered for power without mining new Uranium