There shouldn't be a reason to use ice. The sous vide is a way to precisely control the temperature of the water to keep it constant. So if you want to cook a steak to a internal temperature of 140º you set it for 140º and the cooker will get it to that temperature and keep it there all day. The food will not overcook, it will just stay at the perfect temperature. And the longer you leave it in, the more the muscle fibers will tenderize. When you get home, you just pop it on the grill, or in a pan to brown for a few minutes on a side, and you'll have the best steak.