Hi Clency.
The reduction in land area due to sea level rise is not a threat to life on the planet, although adapting to it may cost humans dearly.
Note that the energy sent out by WTP will be less than that produced today in the furnaces, boilers and engines of electric power plants, factories and vehicles because of much greater conversion efficiency, but all the energy produced by WTP will also eventually turn into heat and this cannot be increased indefinitely.
Much more warming of the atmosphere and water than the heat produced itself is caused by the gases produced in heat production by the so-called greenhouse effect. This alone could lead to the extinction of life on Earth because of too high a temperature, but probably much worse is the effect on life of poisonous compounds released into the atmosphere, water and ground.
For some time now, there has been talk about the confirmed melting of glaciers not on the surface, from the air, but from below, from the land side, which is heated from the interior of the Earth.
Andre