Dumping heat in the ocean is not environmentally friendly. That heat could have been used to heat up buildings.

Almost all the energy we have, ultimately comes from the sun. As long as we can covert energy in a clean way, there is nothing inherently environmentally unfriendly about spending energy on one thing that could have been used for another thing.

Yes, but we need to be careful when we discharge excess heat. The sudden addition of unnatural heat can be an issue for ocean ecosystems. https://en.wikipedia.org/wiki/Thermal_pollution

Something tells me that thausands of heat exchangers where you produce hot and exachange it for cold, at the bottom of oceans which already are recording highest temperatures in modern history, is not much of envrionment friendly.

I would be very surprised if the waste heat had any significant impact more than a few meters/yards from the container. Water has a really high heat capacity and the oceans are large.

Almost all the energy that is heating up the ocean is coming from the sun - we've just put a thicker blanket round the earth to trap more heat. Waste heat from human activity is a tiny part of the global energy balance.