You can see the flat patch of snow- and ice-covered water in the middle of the shot, with many bullrushes beyond leading on to a heavily forested point of land.
In you think this sign is just to prevent some-one swimming here after clearing the snow and cutting a hole in the ice, the sign stays year-round.
The boat launching ramp is heavily used, so Swimmer Danger Number 1 is being run over by a boater, and Swimmer Danger Number 2 is that once you are outside the boat channel, which heads out to the bay and then to Lake Ontario, the bottom is very reedy and unsafe.
So, no swimming anytime.