Several years ago (Before we bought Beachcomber) I planned on towing our 330 Sundancer to the Great Lakes to spend a summer just playing on those waters.
Being as OCD as I was/am I created an Excel file with mileages between cities on the lakes. It doesn't have every little port town listed but most towns that are of any size are included. It's set up like distance charts you would find on any highway map, and the best part about it is it's FREE.
If you want a copy just PM me with your email address and I'll send you a copy. It's only one page so it's not a huge file.