The main reason you need to vacuum seal foods in a bag when cooking sous vide is so that the bag will sink in the water and not float. It’s very important that the food is completely surrounded by the water, that it is fully submerged and not partially submerged so that it cooks evenly. Certainly, the easiest way to do this with most foods is with a vacuum sealer (electric or hand pumped), but there are other ways to do this as well.
Water Displacement Method
If you have liquid in the bag along with the food you are cooking (a marinade, for example), it is difficult to vacuum seal the bag without sucking out all the liquid as well. In these instances, seal the bag almost completely and then lower the bag into some water. The water will displace the air in the bag. When you have lowered the bag almost completely, being sure not to let any water into the bag, finish sealing the zipper lock and the food should sink easily.
Another way to remove air from a bag that has liquid inside is to use your countertop. Hold the bag by the zipper lock and let the food in the bag hang over the edge of the counter. Pull the bag up so that the top of the food almost starts to come over the edge of the counter. Smooth out the bag on the surface of the countertop to remove the excess air and then seal the lock on the bag.
However you seal the bags, remember that a good seal is critical to your success. If any water enters the bag, the food will be ruined and you’ll have to start again.