I live on the north eastern island, in the municipality of Grosse Ile. I don't sail, yet but considering on buy a sea worthy vessel within the next two years.
As Killarney_sailor said, the southern island, Grindstone (Cap-aux-Meules) and Amherst (Havre Aubert) are more popular with visitors to the islands.
But it also depends on what it is your looking for when you come and the time of the year. In June for example, there is the lobster festival where food is a big attraction. The northern Islands have the National and Provincial Ecological and Wildlife Parks. Accommodations here can be slim pickings, though if I'm correct you won't be needing living space.
In July, the beaches and yes I too find the water cool unless you sail into the lagoon at Grand Entry (north island). The bay is bathtub warm.
The Acadian festival starts the first weekend in August and runs for about 10 days which highlights the funniest boat building contest and the largest sand castle event in North America, (So I'm told). Lots of accommodations on all the southern islands.
Music is a given everywhere and food. Islanders like to eat, lol. There are lots of visitor activities during the day and museums etc. to visit during wet weather.