To ensure even cooking, it is important that the ingredients in your bags are fully submerged under the water. A floating bag could lead to uneven cooking and even unsafe food. You also want to keep water from entering the bag at all times by leaving the top of the bag out of the water.
Recommendation Method: Place your ingredients inside the bag, slowly and carefully place it in the water bath with the bag still open. Be careful not to get any water in the bag. The weight of the water will help remove the air in the bag so you can easily seal the bag.
If your bag is still floating, you can put a clean spoon inside the food packet to weigh it down. This will not affect your ingredients while cooking.