Been there a couple times, you have to take a boat that runs up the lake. 100% well worth the trip. (Please ignore the crazy news coming from Alberta these days, most of us are as bat shit crazy as our new premier makes us out to be)
If you ever go also stop at Maligne Cayon which is on the road up to Maligne Lake (where Spirit Island is located)

Also fun fact, Spirit Island isn't actually an island, its only cut off from shore during the spring run off, I can't remember the technical term for this.