You have to fill the space with something, and then mine it away. So imagine if you had a 5x4 pool of water that you wanted to remove. If you filled the water with 20 blocks of material, then mined all of that away, then the space is now empty, the water is gone. Netherrack is a good choice for this since it mines quickly. You should also make sure that new water can't flow in to replace the water you are removing also, block off or dam any other sources.
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There is no special 'swamp water' in vanilla Minecraft. All of the water there is the same as the normal water in the rest of Minecraft.
Clear all the Minecraft folders on your computer and don't use the same Minecraft name :D
I do not know why you would want to clear your XP, but if you do, kill yourself In-Game.
Craft a bucket and Right-click on the water with the bucket.
Hold the bucket in your hand and right click the water.