I’m thinking to do that this summer, did you end up doing it? 

Honey Harbour Boat Club’s Marina offers water taxi services to Beausoleil Island. The prices differ based on which part of the island you are going to. Based on my recent inquiry, they charge $60 one way to get to Cedar Spring docking area (which is where the Visitor Centre and the Cedar Spring Campground are located). The Marina also offers car parking for $10/day.