Well, if going to discuss this, let's discuss it with some meat to it.
Source:
http://www.osl.gc.ca/en/donnees/monitore.htm#temp-sal
These are average temperatures from 0-98 ft (I know, 98's a weird number, the page is in metric and it's 30m), taken for a survey for shrimp, of all things. Dates are late summer/early fall
06-Aug-1998 and 25-Sep-1998 - 50.3°F average
05-Aug-1999 and 29-Sep-1999 - 51.5°F average
05-Aug-2000 and 28-Sep-2000 - 52.2°F average
03-Aug-2001 and 28-Sep-2001 - 49.7°F average
So, that give broad-sprectrum for late summer for the entire Gulf of St Lawrence.
Also found an article about a swimmer who drowned in the harbor a few year ago, on Aug 16th, surface temp was 57.2°F, which gives a good mark for the specific area that I'm in.
If anyone wants a map, here's a link (picture will just eat up bandwidth):
http://www.etourist.ca/maps/pei.htm - area I'd be interested in to try for something would be the Northumberland Straight between the island and the mainland (specifically right next to the big 9 mile long bridge, that's where my cottage is)
Was figuring on about a 60°F for a tank temp... would be in the basement, so figure that'd be about the right temp down there.
*disclamier - work is in Christmas slowdown, no one calling for tech support, so I'm bored, and doing stuff like this keeps me occupied