Hey!
Hope this helps...
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The water that makes up icebergs, Antarctica, and such... is water that has stayed in the same place for 100s of thousands of years. Now this water froze and stayed at that height, because the water was at that level before (long ago), you have to remember that at one point in time, it is believed that the Earth was covered only with water, and had no land, besides for the bottom of the water.
Now the water that makes up the surrounding ocean, is water that never froze, and is constantly renewed. Plus it never has the time to freeze, so it cant stay at the specific level. Additionally, the water that makes up the surrounding ocean, is constantly going under water evaporation, and precipitation... Plus the water from those oceans are salty, meaning they can't freeze, and if you cant freeze, you can only melt... so any water from the ocean that freezes, only melts in return...