Yes you'd 'need' that kind of land, but it's not like that land is unusable for anything else - you can still farm it, have roads, facilities, etc...

As to offshore, you'd be surprised by how much "scope" there is, even with such depth limitation. And 30m is only a limit right now at current power prices and in the face of current subsidy regimes for coal, gas et al.

In the long run, we're all dead. John Maynard Keynes