you shouldnt hear water slopping around in the water heater, it should be full
pressurize the water system and open a hot water tap until you get water with no air.
check that hot and cold are hooked up the right way on the heater!! cold comes in at the bottom. crack t&p relief valve until water comes out
BUT REALLY IMPORTANT, make sure the heater is hooked up correctly, cold water in at the bottom, hot out at the top, if its backwards, the tank will not fill completely and im sure its been done wrong before. if its not filled completely, you wont have much hot water of course but even more important, the electric heater may not be submerged completely in water and it WILL burn out