The problem I'm concerned about isn't raising the average temperature of the ocean. The problem I'm concerned about is localized heating. Marine life is far more vulnerable to rapid temperature changes than life on land. We've seen dead zones developing in the past downstream of power-plants, as the warm water discharge from the steam turbines raises the temperature of the water and reduces its oxygen carrying capability. My concern is that if we put datacenters in the ocean, they'll create localized hot-spots with much the same result.