Oh and you got your "wetfoot" term very much incorrect - its a different one and its normally used in a degrading and inflammatory manner.
I think your thinking of the term "Wetback" that is as old as the hills and is indeed a very derogatory name for Mexican immigrants.

Main Entry: wet·back
Pronunciation: 'wet-"bak
Function: noun
Etymology: from the practice of wading or swimming the Rio Grande where it forms the U.S.-Mexico border
usually offensive : a Mexican who enters the U.S. illegally