Laboratory data on supercooling and frazil-ice formation in sea-water are analysed using a boundary-layer model. The model is based on a turbulent channel-flow boundary-layer theory, in which buoyancy effects become important because of vertical gradients in temperature, salinity, and suspended frazil-ice crystals. The frazil-ice crystals are treated as thin uniform plates. By assuming a mean face diameter, a mean thickness, and a mean Nusselt number of 10−3 m, 10−4 m, and 4, respectively, the general experimental findings are well reproduced by the model.