The leasing cost is buying land (ammortized over 30 years) + maintaining land and then compare it against buying specialized equipment, hardware, and maintenance staff for underwater projects.

I guess with scale, the price goes down.

I'd assume you still need a small datacentre close to it, i.e. on the cost, for routing, backup generators, maintenance, etc.

"Small" being the operative word. A room vs. a building.

Well, that depends on your definition of small. This is what I believe the diesel generators of a typical datacentre look like (+fuel tanks):

http://www.dchuddle.com/wp-content/uploads/2014/08/Backup-ge...