In early spring I would not be more than 10 minutes from shore if I tipped.  You are not going to be able to right a swamped canoe and you will not be able to pull a canoe to shore even if it is only 50 yards from shore.  I would be very careful in early spring conditions.  The next time you go to Quetico in the early spring go for a swim and see how long it takes to get really really cold.  It will be less than 5 minutes.