A following sea refers to a wave direction that is similar to the heading of a waterborne vessel under way.
The word "sea" in this context refers to open water wind waves.
It is also used during ceremonies, such as the beginning of a voyage, a ship's commissioning, a retirement, funeral et cetera.
[2] Following seas, combined with high winds (especially from the stern, or from behind the boat), can be dangerous and cause a boat to yaw (turn sideways) and swamp or plow under the wave ahead, if the winds and sea are too strong or violent.
However, in the mariners' traditional toast or blessing a "following sea", combined with a "fair wind", to a sailor, implies that the winds are comfortable, the sailboat is "running", i.e. sailing with the wind on its stern, and the seas are comfortably rolling in the same direction as the boat is heading, so that the boat seems to be skimming easily on the surface of the water.