The phrase "water column" refers to the vertical space or column of water in a body of water, such as an ocean, lake, or river. It includes all the water from the surface to the bottom. Full definition
Before ice can form on the surface, the entire water column must first reach 40 F, which is likely to first occur along the shoreline. (judithcurry.com)
The major one looks to be additional latent heat provided by evaporation which is being caused additional back radiation and the warmer water column. (tamino.wordpress.com)
I did not suggest that the whole water column needed to reach a new temperature. (judithcurry.com)