Andrew...I didn't know Sunsail was around here. Most people that I run into use Desolation Sound Yacht Charters
It's a local company and I have never heard anything bad about them.
As far as local knowledge goes, it helps quite a bit but I wouldn't say it's necessary. Desolation Sound isn't too bad/tricky to figure out as it's well charted. Beyond Desolation Sound heading north and into the inlets and through the various rapids definitely helps with local knowledge but having said that, "Sailing Directions" is the bible for this and pretty much has everything required. As a matter of fact it is a requirement to carry this Canadian Govt. publication on the vessel along with the paper charts of the area your in. I like the "Dreamspeaker" and the "Douglas and Hemingway" cruising guides but again "Sailing Directions" is the key along with a good tide/current table.
I think Erps knows Roscoe Bay like it was his back yard, I've only been in there once that I can remember.