There are charter companies in Powell River and Campbell River. Whether any of them have a GB in their fleet I don't know. The people I am familiar with who have chartered boats from these two locations did not get GBs.
If one is interested in specifically chartering a GB for a cruise to Desolation Sound or anywhere else between here and there they cannot do better in terms of the boat than Northwest Explorations http://www.nwexplorations.com
. While getting to Desolation Sound from Bellingham takes a few days longer than getting there from Powell or Campbell River, the journey is well worth making in my mind. Lots of interesting places to visit along the way or you can simply "blast" straight on up there.
My only connection with that company is that we bought our old GB with the assistance of the fellow who is today their lead broker but in 1998 was working for the predecessor of the company, Grand Yachts Northwest. And we walk by their fleet every time we go to or come from our own boat. It is about as good a selection of GBs that are maintained in outstanding condition that you'll find anywhere.