I hug the shorelines when paddling in early spring and late fall, close enough that a VERY SHORT swim will get me to where I can stand up and get out of the water.  If you can go along a lee shore it is also much easier paddling.  If in heavy wind, being able to see your way out of the situation quickly, if needed, goes a long way to making you relaxed and confident in the waves.  No matter what season, it takes a very long time to paddle or swim a swamped canoe any distance to shore.
Regarding timing:  Mid-June to the end of August water temps are bearable.  Keep in mind that darker lakes and shallow areas, and rivers, are warmer in general and big, deep and windier lakes will be colder even in mid-summer.