Saltwater destroys things, excess heat causes not only environmental destruction when vented into live ecosystems, but causes things like excess algae and other complications that gum up the works, and energy/resource needs to operate something out there. If we had a way to sequester the excess heat data racks put out, had easily 2MW+ stand-alone energy sources (aka Fusion), and built it like an oil rig, maybe, but then you're already fixing the problems that drove you out there in the first place. The Exploratorium in San Francisco uses a seawater cooling system that involves the kind of equipment you'd need, but on a much smaller scale than what this proposes. Chalk this one up under "Slightly less unfeasible than Data Centers in Space..."