I remember Microsoft experimented with this back in 2020, and while they said it saw successes, it doesn’t appear they’ve done much after the proof-of-concept.

From 2024:

In a separate statement, Microsoft told DCD: “While we don’t currently have data centers in the water, we will continue to use Project Natick as a research platform to explore, test, and validate new concepts around data center reliability and sustainability, for example with liquid immersion.”

Last year, China’s Highlander deployed a 1,433-ton commercial underwater data center off the Hainan island coast.

but at what cost?

I heard about this. I once asked Deepseek if it’d rather be cooled by molten salt and it said it’d be overkill.