Maybe it's from わたしは (watashi wa - I am)? They may have chosen to use "watasha" rather than "watashi wa" to fit the meter better, or to better fit the folksy feel of the song. But I'm not sure.
I have never heard the word "Watasha" being used as meaning "I". Is that a dilect change or somethng else?
-- by pinky01
at 2011-10-23 09:05:16