The latent heat of freezing, or the enthalpy of freezing, for the phase transition from water to ice is 334J/g for pure water and just slightly lower for sea water.
The energy (heat) released from the freezing layer of water is conducted through the ice and released into the atmosphere.
The thermal properties of the ice and snow cover, as well as the energy balance at the top and bottom of the growing ice sheet determine the rate at which heat can be pushed out.
For an ice surface that is in thermal equilibrium with the atmosphere, conservation of energy mandates that the heat must flux in and out of the ice .
This can be expressed mathematically, and using calculus techniques the thickness to which an ice sheet can grow can be predicted for given surface temperature conditions.
On relatively calm waters, one of the first forms of ice to form is this slimy looking ice called nilas. Photo courtesy Rolf Gradinger.