At some point on your way to Antarctica you'll pass over the Antarctic Convergence, also known as the Antarctic Polar Front. The ocean south of the convergence differs greatly from northern waters in salinity, density and temperature. A great mixing occurs where northern and southern waters meet, and nutrients from the seafloor are brought to the surface, making the convergence a highly productive area for algae, krill and other small creatures at the base of the Antarctic food web.
The exact location of the Antarctic Convergence varies slightly throughout the year, and also from year to year. Despite what you may hear, there’s very little sign that you're crossing the convergence. The sea does not get rougher, and there's usually no change in its appearance. The primary indicator is a dip in the water temperature, a change that the ship’s instruments will detect but that you almost certainly will not.