When is water more buoyant - in winter or summer?

The density of water increases as the temperature decreases and approaches a maximum at 4oC. After this point the density decreases until 0oC at which point the water changes to ice.

The density and the buoyant force of the water therefore increases until the 4oC point at which time it decreases slightly:

Temp ( °C ) : Density (g/cm3)

0 : 0.9999

4 : 1.000

20 : 0.9982

40 : 0.9922

60 : 0.9832

80 : 0.9718

Using Lake Superior as an example, the surface water temperature varies between 20oC July/August and slightly above 0oC in December/January. The water provides 1% more buoyancy (it's density is 1% higher) in winter.