I wonder why they are submerging these instead of just making them a floating barge.

I can't imagine submersion is significantly better for cooling, the energy density very likely already requires a circulating water system, and the added issues dealing with the pressure at depth seem risky.

Possibly there is a benefit not related to cooling. Perhaps weather/waves? Things are probably alot calmer 30ft below the surface. The tossing and turning on the surface may place additional streses on things like HDD spidles.