Platelet ice is the name given to ice crystals that nucleate in the ocean and grow either at depth or loosely attached to the ice-water interface. Related to the proximity of ice shelves, it is known to form in supercooled seawater. This study details the conditions for its growth during the austral winter of 2003 in McMurdo Sound, Antarctica. A key finding is that the presence of platelet ice in the sea ice cover is conclusively linked to the time history of the appearance of ice crystals in the water column, monitored using the strength of the backscattered signal from an acoustic Doppler current profiler as a proxy. Generally, these crystals appeared from mid-May as water near the interface became supercooled. This near-surface supercooling appears to be related to a simultaneous, abrupt change in the structure of the upper 250 m of the water column, from one that behaved dynamically and contained both the warmest and coolest measured temperatures to one in which the water was essentially isothermal near its surface freezing temperature. Near-surface ocean temperature was also affected by tidal mixing and by an increase in the thickness and density, over the course of winter, of the surface mixed layer created by salt rejection during ice growth. These processes allowed cold water, trapped by buoyancy in a band at the base of the mixed layer in early winter, to gain access to the ice-water interface by midwinter. This band of cold water probably had its origin beneath the McMurdo Ice Shelf.