According to footnote 121 in Chet Haase's book 'Androids' Bob had used this nickname since highschool and even used it as his corporate email address.

Does not answer where it originally came from, but he seems to have enjoyed the nickname.

I read that it originated from his waterpolo days.

can confirm. It's a nickname he earned as a teen playing water polo.

That actually makes a lot of sense.

I half expected we would find it on his long form birth certificate.