Yes, there are public washrooms in a few locations:
Visitor Information Center, downtown at 521 First St W (just east of Fifth), open 7 days a week 10am-5pm.
There are also washrooms open daily May to September, weather permitting, at the Cochrane Ranche Historic Site (Hwy 1A east of Hwy 22) and Mitford Pond Park (west end of West Rock Road off of Hwy 22).