The water is still warm in early September - well above 55.  I think you can be in it a long time if you have to be.

Whether or not to lash packs in remains an unanswered question for me.  In terms of packs getting waterlogged - if you have a plastic liner in them, won't they float for a long time?  I know water will eventually find its way in, but it would take a while, and it sure would be nice to have packs with you and the canoe when you got to shore.  

At the end of July I was paddling a solo (borrowed from Solus - thanks Jon!) from the cabin and went for an unplanned swim.  It was a very windy day but I was on a small lake and the waves weren't bad.  I was testing my abilities and how the boat would respond and was crosswise to the wind, which grabbed the boat and send me tipping  over.  I'm still not sure exactly what happened, but it was alarming.  The really stupid thing was I did not have my PFD on.  I was pretty close to shore but I wanted to get there with the canoe because I couldn't walk back from that part of the shoreline (and I wanted to give Jon his canoe back).  I managed to get my PFD on - very difficult - and then pulled the canoe to shore while it was upside down in the water (it wouldn't stay upright).  I was fully clothed including with a rainsuit, but was still worried about getting chilled in the cold strong wind once I was out of the water.  However, once I got the boat emptied out I was able to paddle the mile or so home with no problem.  I was working hard and it kept me warm.  It was a pretty chastening experience.  In a lifetime of paddling, I've never unintentionally flipped a canoe on a lake before.  It definitely made me appreciate how hard it is to get to shore, so I'll probably be more cautious in rough conditions in the wilderness going forward.