While cold seawater is, indeed, much better at cooling than warm water, it is less efficient for our refrigeration systems. Having the cooling water be at about the condensing temperature of the refrigerant is ideal. Typically, systems are most efficient with 90 degrees F / 32 C sea water (varies with the refrigerant). At much lower temperatures, like 55F/13C up north, the system will operate with a lower suction pressure and the pressure across the expansion valve is reduced which decreases system capacity and increases running times. We've got a gate valve on the sea water outlet that can be partially closed in very cold water to restrict flow and increase temperature. Put your hand on the drier or receiver and it should be about body temp. Recirculating the fresh water in your tank instead of using sea water is great and has the added benefit of keeping the beer cold when you're on the hard. 
Cheers, Craig