Freezer bag = vacuum sealed?
You do not have to seal the bag, in fact as you lower the steak in the freezer bag in the water the air in the bag goes out. You need a couple extra inches of bag so you can clip it to the top of the pot or container, no water gets in. The steak in the bag is completely submerged in the water except the top. I didn't get it until I did it the first time. The steak can also b e completely sealed in the bag but I don't want the opening of the bag to be in the water sealed or not.
@temery, you can do some sous vide cooks in regular zip lock bags that are "sealed" via
water displacement. Other cooks, such as for longer time periods, stuff you want to be extra careful with, need to submerge completely, or things you plan on saving for prolonged periods, should be properly vacuum sealed.
I have both a vacuum sealer and also use ziploc bags, because the vacuum bags are more expensive. I do, however, usually zip the ziplocs after removing the air, to keep stuff from floating or accidentally getting water in there (especially for lighter items like sausage).