First thought as well. I mean, those datacenters on land also are pretty bad (especially compared to how "useful" they are in the most common cases), but that thing in the sea is somehow a bit more scary for several reasons.

But then I'm thinking: there are volcanos deep in the ocean, and life around them is surely different than in colder areas, but it doesn't seem to have much impact on larger area. It surely depends a lot on where you place your datacenter, but, theoretically, it should be possible to make it harmless.

The other thing is that of course tech giants placing their datacenters under the water won't care a bit about correct placing, and in the end there won't be anyone to stop them.