Although I have not kayaked the Slates, it is not because I did not try. Both days that we had available to try the Slates were to foggy to cross from the mainland to the Islands. When we inquired about shuttle service, the price shock almost knocked us to the floor. $500!!! You figure that they are taking you there and then going back and then returning to pick you up and then taking you back to the main land; so 4 times, they have to burn gas. Very expensive. You are better off to paddle for sure, but it can be testy. To be honest, the best sea kayaking that we have ever done was about 15 miles to the West of the Slates at Rossport. The Rossport Islands are breath taking. Most of them are protected.
We stayed at the Rainbow Falls Provisional Campground. Fantastic location because there are at least 4 islands that you can get to right from your campsite. Seeing that you are only 15 miles from the crossing for the Slate Islands, you can plan your trip to the Slates by the weather. If its going to be sunny and calm, then drive the 15 miles and do the open water crossing to the Slates, if the weather is a little testy, then do some of the Rossport Islands.
The best time of the year to go is the last week two weeks in July, through the first two weeks of August. We had unbelievable warm weather and Lake Superior was 70 degrees!!! We snorkeled every day.
Have fun and enjoy the most beautiful sea kayaking in North America!
Pat from Rockford, MI