Originally Posted by Jayvis

You're not taking into account the difference in density between fresh water and salt water. Typically an object immersed in a fluid is buoyed up by a force equal to the weight of the fluid it displaces. However, freshwater is not as dense as saltwater, freshwater actually has greater volume than an equivalent weight of saltwater. Thus, when freshwater ice melts in the ocean, it contributes a greater volume of melt water than it originally displaced.