With enough data centres like these, all the excess heat will go straight into the sea, further contributing to climate change. The only place these data centres wouldn’t contribute further to climate change is if they were buried deep down in earth’s crust but then cooling would be an issue.
I think I prefer the idea of city center data centres where the excess heat is put to use for central heating systems
Others have covered the fact that the reduced power load from lack of AC almost certainly outpaces this, but also, it's...really unrealistic given the size of the oceans. There are over 1.3 trillion liters of seawater in the world. It takes 4,200 joules to raise it by one degree. That's 5,586,000,000,000,000 joules.
You could put 100,000 of these in the ocean, each putting off 100,000 joules a year for 100,000 years, and it would have raised the temperature of the ocean by less than 2/10ths of a degree (celsius).