It isn't well known for fishing. Most go to Pender for other reasons. Problem with most of the Gulf Islands is lack of rivers/creeks on most. Some salt-chuck fishing in a boat, but trolling or mooching deep. I had never heard about lakes there, but apparently they exist. Another site mentioned that at least one lake has trout, but it is private.
Saltspring Island would be the better bet. No matter where you are on the Gulf Islands, this time of year is too warm for the few places there is fresh water, and generally too rocky (ie. cliffs) for fly fishing. I live on Vancouver Island and Saltspring is the only one I have bothered with.