If you really have to ride in the wet; List of trails that are best able to take it.
http://mtbtrails.ca/wet-weather-approved-trails/789 (additions/deletions to the list appreciated by Wade/Sharon)
Well-written article by Steve Sheldon from TORCA http://www.torca.ca/riding-in-the-wet-a-message-from-torca-trail-director/
Another well-written article about general considerations http://www.singletracks.com/blog/trail-advocacy/opinion-stop-riding-like-this-ease-the-impact-of-mountain-biking-on-the-environment/
This is just for informational purposes. Not intended to point fingers or criticize.
Trailwork video for entertainment purposes