I was born in Halifax. I've lived in Montreal, Toronto and New York. Consequently, I can no longer remember the preferred pronunciation and sometimes spelling of certain words such as process, route, pasta, cheque etc., but I do know that no one in Canada says aboot. It's more like a-boat.
Here is an explanation I stole from a Yahoo. "Non-CDNS will hear "house" as a 'funny' word, because the vowel 'ow' is followed by a voiceless consonant: s. The vowel changes to the 'uh'-ish sound - as in, 'The criminals house fugitives on a regular basis.' Non-Canadians will not hear 'house' as a funny word, because the vowel 'ow' is followed by a voiced consonant: z. The vowel, therefore, doesn't change.
"The reason that this difference appears to be so major is because the new vowel that is produced does not exist in many other varieties of English. Therefore, when hearing this strange vowel, people who don't use it themselves will try to match it to the next closest one they know, which is 'oo' as in 'boot.' So, Canadians don't actually say 'aboot' and 'hoose,' it just seems like that to the ears of those who aren't accustomed to hearing that vowel."
If you have read and understood all of this explanation, you can be my friend.