So I've had to follow a rabbit trail many others have had to lately, and figure out admissibility to Canada with a DWI arrest in my past. Finding the search to be quite frustrating (albeit not undeserved, because in the end I did put myself in this situation), I thought I'd compile what valid, or at least credible, information I could in this post.
Government References:
1) Port of Entry Examination Procedure
http://www.cic.gc.ca/english/resourc.../enf04-eng.pdf
2) Procedure for Evaluating Admissibility
http://www.cic.gc.ca/english/resourc.../enf02-eng.pdf
3) Canadian Border Services Website
http://www.cbsa-asfc.gc.ca/menu-eng.html
4) Admissibility Criteria
http://www.cic.gc.ca/english/informa...lity/index.asp
Additional information is from attorney websites. One key piece of information - that the FBI NCIC database is not searched at primary inspection, but is searched at secondary inspection, comes from multiple posters who have claimed to be CBSA officers on both FT and Reddit. I could not find an official source for that, but did verify the posters weren't "trolls" via their other posts and the info in the above reference materials does not point to searching criminal databases at primary inspection.
The easy basics:
(info from Ref 4). A conviction of DWI (or if there's reason to believe you committed such an act) will leave you inadmissible to Canada. You can be deemed rehabilitated at the conclusion of 10 years after your sentence is completed or can apply for rehabilitation after 5 years. Without rehabilitation, a Temporary Residence Permit can be applied for, but your need to enter Canada must outweigh the risks.
One layer deeper:
(info from Ref 2, with additional interpretation from some attorney websites). If your court proceedings are over, and the final disposition is "not guilty", or "dismissed", or pretty much anything other than "conviction" you are likely admissible to Canada. For the US, many states have a pre-trial diversion program that would land you in these categories. Expunged records from the US should be honored at the border.
For Secondary examinations at the border, Canadian officers have access to the FBI NCIC database, and that database may not be updated with the final disposition of the case. For this reason, you should have all court documents at the border. An arrest without a documented final disposition will leave you inadmissible. This can either be due to the trial not having occurred yet (in this case you're still inadmissible due to having reasonably "committed an act"), or due to your local court information of "not guilty" not making it back up the FBI chain. Either way, as a foreign national the burden of proof is on you.
Deeper Yet
(Info from Ref 1, Ref 3). At primary inspection, your criminal history should not be asked about (Ref 1 pg. 29-33). The reject rate of people at the border seems to be < 0.5% of all travelers.