“The amount of natural tritium is therefore negligible and to get tritium you have to breed it. “
False. The amount of tritium in the oceans is estimated to be about 26.8 ± 14 kg.
Tritium is also produced by our existing fission power plants.
Exactly. You have to breed it. And the point of fusion power is to avoid unclear fission power, but you need fission to breed the fuel for fusion.
And no one recovers tritium from seawater because it is so dilute that the cost of recovery outweighs the benefit. And furthermore the inventory of tritium in the environment is mostly the result of nuclear testing and greatly outweighs the tritium that exists "naturally" as the result of cosmic ray generated neutrons activating deuterium in seawater.