> Sizable’s reservoirs could connect to any grid that’s near waters that are at least 500 meters (1,640 feet) deep.

How many big cities are there on earth with that depth available nearby?

It depends on the definition of "near", but there's a sizeable population within ~40km, which is a reasonable distance for an offshore wind-farm.

Almost the entire Mediterranean is >500m depth within just a few km of the shore, and that's half a billion people. All of the eastern seaboard of the North+South American continent is available at 100km distance (another 100-200mn people). Most of west Africa, all of Australia, and almost all of the western flank of the Pacific.

Maybe a quarter of all people live within 40-50km of a 500m deep sea. Definitely a large TAM.

Could also be interesting in case the idea of under water data centers ever returns

https://en.wikipedia.org/wiki/Project_Natick

Or it could help offshore wind farms provide a more stable/predictable output.