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Yes its still pretty cold in may considering its the spring.. I havent seen anyone swimming .. besides you wouldnt want to go swimming in lake ontario its very dirty. Lake Erie would be your best bet for your location. the water and sand is nicer.. try planning the trip for june/july?

Its verrrryyy cold during may and like the person said its also very dirty warmest itll be will probably be in august towards the end of the summer an its usually the warmest around here during august so yeahh