The main fusion reaction being worked on is deuterium-tritium fusion. Deuterium is found naturally and as mentioned makes up less than 1% of the hydrogen in water. Tritium is manufactured in nuclear reactors. Proton-proton fusion, using normal hydrogen, isn't under consideration because the reaction results in a diproton which almost always splits up again, only rarely beta decaying to deuterium. But even if it was, annual world energy use is on the order of 1018 Joules. That entire energy demand could be met by a single ton of hydrogen, so 10 tons of ocean water. The world's oceans contain a billion billion tons of water. Maybe one day we'll (f)use it all up, but by that point Earth will be a likeness of Coruscant and humanity will be an interplanetary if not interstellar civilization. And what's valuable is fresh water. Seawater is not scarce for any non-landlocked country, but isn't very useful (can't drink it, can't water crops with it) and it's expensive to desalinate it.