Species identification. Is this a leech?
Yes this is a leech. It appears to be a species in the family Glossiphoniidae, or the freshwater jawless leeches. This family of leeches is relatively flattened with a poorly defined anterior sucker.
According to The Canadian Encyclopedia, there are about 45 species of leeches found in or around Canada.
Langer et al. (2018)1 provide a more limited selection of Glossiphoniidae leeches they ascribed to Ontario. Of the ones they list, yours most looks like a specimen from the genus Placobdella.
See, for example, this image of Placobdella phalera from de Carle et al. (2017)2 found in Ontario, Canada:

 Source: Figure 1, de Carle et al. (2017) 

1  Langer, S.V., Vezsenyi, K.A., De Carle, D., Beresford, D.V. and Kvist, S., 2017. Leeches (Annelida: Hirudinea) from the far north of Ontario: distribution, diversity, and diagnostics. Canadian Journal of Zoology, 96(2), pp.141-152. 
2  de Carle, D., Oceguera-Figueroa, A., Tessler, M., Siddall, M.E. and Kvist, S., 2017. Phylogenetic analysis of Placobdella (Hirudinea: Rhynchobdellida: Glossiphoniidae) with consideration of COI variation. Molecular phylogenetics and evolution, 114, pp.234-248.