Where does the water go? Assuming you want to reduce sea level by a 1/2 inch using this mechanism, you have to do the equivalent of covering the entire ETA: land area of earth in a full inch of water (what's worse, seawater; you'd want to desalinate it). Even assuming you can find room on land for all this water and the pump capacity to displace it all, what's to stop it from washing right back out to sea? Some of it can be used to refill aquifers, but the capacity of those is trivial next to that of the oceans. Some of it can be stored as ice and snow, but global warming will reduce (actually, has already quite visibly reduced) land glaciation; even if you can somehow induce the water to freeze, that heat you extract from it will have to go somewhere and unless you can dump it out of the atmosphere entirely it will just contribute to the warming. The rest of the water will just flood the existing rivers in its mad rush to do what nearly all continental water is always doing anyhow: flowing to sea.
Clearly, the solution is to build a space elevator and ship water into orbit. We lower the sea levels, the water is there if we need it later, and in the meantime we get to enjoy the pretty rings.
(No, I'm not serious.)
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