Not a subsea engineer, but having worked in the space previously I can tell you that we already have giant man made heaters sitting at the bottom of the ocean. Subsea Oil and Gas wells take advantage of the steady temperature at those depths to dump excess heat form the production fluid (sometimes up to 120F for straight crude, much higher for oil and gas mix) whilst and before it rises up to the platform. Sometimes using active or passive cooling on the ocean floor.
Then there's natural geothermal vents doing that, but probably an order of magnitude more, since before humans were around.
Then there's natural geothermal vents doing that, but probably an order of magnitude more, since before humans were around.
The ocean is a pretty big thermal mass.