The tilde, ~, generated by the U.S. International - PC layout and the tilde, ~, produced by the U.S. layout are allegedly different characters.
How can I produce the U.S. tilde while using the International keyboard?
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Take a look at KeyRemap4Macbook if you want to permanently remap, it's got a tonne of options.
Another way to do this, if you're using Lion or above is adding to the
PressAndHold IOS-esque feature.
Ukelele can allegedly do this, or take a look over here: Mountain Lion - Umlauts with english keyboard layout