Setting aside a small patch of ocean for the task seems like a much better plan than the current practice. Provided you dump it in a place with a decent current any adversely affected area should be exceedingly small.
Keep in mind that the sun is constantly dumping energy on us. Absorption averaged across the entire earth is ~200 W/m^2. Assuming I didn't misplace some zeros somewhere then a gigawatt corresponds to ~5 km^2 of ocean surface. That's the daily flux. Penetration falls off exponentially so 75% of that only ever makes it ~10 m down.
I think the takeaway here is the utterly incomprehensible scale of the ocean.
Keep in mind that the sun is constantly dumping energy on us. Absorption averaged across the entire earth is ~200 W/m^2. Assuming I didn't misplace some zeros somewhere then a gigawatt corresponds to ~5 km^2 of ocean surface. That's the daily flux. Penetration falls off exponentially so 75% of that only ever makes it ~10 m down.
I think the takeaway here is the utterly incomprehensible scale of the ocean.