Temperature is what matters. The same amount of heat out into a river can significantly warm the whole river downstream, but have no detectable effect on the ocean temperature, just because the heat is diluted so much.
Like, dilute a bottle of tequila with two liters of cola, sure you'll get drunk - but pour that same tequila into a full swimming pool, and you'd never get drunk even if you drink the pool water all day.
You wildly underestimate the size of the oceans. If they're going to be built anyway, and if they can have a long lifespan even in salt water, this would be far better for the environment than the currently popular method of using rivers for cooling - rivers can be warmed by perceptible and significant amounts until they carry that extra heat into the oceans, plus fresh water is more rare, precious, and in need of protection than salt water.
About half of solar energy passes through the atmosphere, that factor of two isn't a good enough reason to use space-based solar panels for ground-based power grids.
Even though the microwave receiver power station in SimCity 2000 looked so cool.