I rarely ever get to the coast, so most of the places I know about are in the southern interior. However, looking at Google Earth, I see some promising looking places inland from Powell River.
For starters, check out the small lakes, ponds and meadows/bogs around
http://maps.google.com/maps?t=h&hl=en&i ... 19097&z=15 for instance, not to mention other surrounding places. Check out every body of water and meadow/bog in the area, and you should hopefully find something. I notice that there is a large lake south of Horseshoe lake, which appears to have water levels that vary. When the water levels drop, there could be lots of mud exposed! Also, where streams/rivers enter could be worth exploring, such as at
http://maps.google.com/maps?t=h&hl=en&i ... 38195&z=14 .
There are lots of places to explore, and there seems to be roads close enough to get to most areas. But, early in the season, things may be a bit more submerged, so places that aren't good right now may be excellent by August! But, you never know, until you start looking!
I don't know if there are any tidal flats in your area, but they can be a great source of muck at low tide, but special care has to be taken, since the last thing you need is to get stuck with the tide coming in!

Also, places like that may be more in the open, and visible from vessels and stuff, unless there are any salt marshes with hiding places. But, I don't know the area beyond what Google Earth (and Google Maps) can give me, and it looks like a lot of shoreline seems populated, at least where there is aerial photo coverage south of Powell River.

But, a lot of the shoreline around Powell River is in lower resolution satellite imagery, so I don't know the extent of the populated areas in those regions due to lack of detail. But, inland, such as the areas I gave links to, seem a bit more isolated, and your best bet.
Good luck!
