"There are 1,335,000,000,000,000,000,000 liters of water in all the oceans."
"A large fusion power plant, generating 1,500 megawatts of electricity, would consume approximately 600 grams of tritium and 400 grams of deuterium each day. A 1,000 MW coal-fired power plant requires 2.7 million tonnes of coal per year, while a similar fusion plant would need only 250 kg of fuel per year (half deuterium, half tritium)."
You do the math. I'm personally not worried about running out of water due to fusion.
"A large fusion power plant, generating 1,500 megawatts of electricity, would consume approximately 600 grams of tritium and 400 grams of deuterium each day. A 1,000 MW coal-fired power plant requires 2.7 million tonnes of coal per year, while a similar fusion plant would need only 250 kg of fuel per year (half deuterium, half tritium)."
You do the math. I'm personally not worried about running out of water due to fusion.