"Watashi" is a Japanese word corresponding to "I" in English.
"Watashi" does not imply femininity; both sexes use it in
ordinary conversation. "Boku" is used primarily by men and
boys, but can sometimes be used by women. Much is often made by Americans and other foreigners
of gender-specific speech and usage in Japanese, but much
of this is overrated. Read _Japanese: The Spoken Language_
volume 1 (Noda) for more information.

the female use of "watashi" in speech indicates a traditional societal prejudice towards a more proper behavior for women.

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Watakushi: The most formal way of saying I.
Watashi: Less formal than above.
Boku: Used in informal conversation. Usually used by males only, although some hip girls are using it nowadays.
Ore: Used in even more informal context. Again, males only.
Watashi is the word for 'I' first taught to people learning Japanese.
The difference in male and female speech is only in informal speech. It's like in English: A female using male speech is considered, uh, 'butch', and a male using female speech is considered 'a fruit'.