Hornby Island is a significant site for petroglyphs, many of which can be seen from the water. You can find a description of the petroglyphs' locations in Doris Lundy's famous thesis,
The Rock Art of the Northwest Coast, available for download
here. The Hornby Island petroglyphs are at document page 52, PDF page 62.
Better yet, if you can get your hands on a copy of Beth and Ray Hill's book,
Indian Petroglyphs of the Pacific Northwest, you can find a precise map and photographs of the petroglyphs, at pages 125 (map) and 130–137 (photos). See if your local library can get you a copy of Hill through inter-library loan. If that's not an option, PM me through the forum.
Nearby Chrome Island is another major petroglyph site. Again, consult Lundy and Hill. Supposedly, the lighthouse keepers on Chrome Island will show you the petroglyphs if they think you're cool—which you definitely are, so no worries!
The local artists—ie, basically everyone on Hornby—know all about the petroglyphs, so whoever you're staying with should be able to help you find some. In fact, a lot of the artists incorporate themes from the petroglyphs into their own artworks!
Edit: Here's a link to an
online scan of Beth and Ray Hill's book,
Indian Petroglyphs of the Pacific Northwest. Just wait for the ads to finish, and you can click through. The Hornby Island petroglyph map is at document page 125, PDF page 129 of 328. Photos of the Hornby Island petroglyphs are at document pages 130–137, PDF pages 134–141 of 328.
Alex