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NOAA has some good data for water temperatures. You can go to NDBC - Observations - Radial Search and it will search from the latitude/longitude for data. Once you get a list of data sites, you can click on the link to figure out exactly where it is (some might be on an inland lake, some might be on the St. Lawrence). I gave coordinates for a point on the St. Lawrence to start from.
I think most of them will be out in Lake Ontario. The St. Lawrence is usually warmer than the lake.