It's somewhat obscured, but I suggest a northern water snake (Nerodia)

I just wrote out a big long answer discussing things like Racers, Watersnakes and Black Rats discussing the pros and cons of each......and then it occurred to me what it actually is!

This is a melanistic (black) Eastern Gartersnake (Thamnophis sirtalis). The melanistic (black or black-backed) morph is common in some areas in the north of the species range.

@fhfchrish, Could it be a Black Racer?

It could be racer, but I'm pretty sure it isn't. That was my first answer I was trying to expound upon above when it hit me that it was a gartersnake. The colors and scalation of the head are right on for melanistic Eastern Gartersnake which are known to occur at various locations in Ontario.