Although most companies provide snow boots and some even offer a gift coat, it is important to travel with the right clothes. Think that Antarctica is very cold, even in summer, and it is better to be well prepared. The essential is to dress in layers. This is equivalent to saying stockings, shirts and thermal pants to start dressing.
Then carry snow pants, hats, scarves, waterproof jacket with the higher the waterproof range, the better, hooded parka and better if it covers the waist, for the moments when you have to be sitting on the zodiacs, gloves. Take a bathing suit too. Although it is usually amazing, it is possible to bathe in some islands and that is an experience that must be had, even once in a lifetime.
If you do not have all those clothes, you do not need to put on one more expense. In Ushuaia you can rent snow clothes prepared for these trips.