This depends on your baby’s height. The first time or two, fill the bathtub with enough water so that their feet barely touch the bottom of the tub when they’re fully upright. Being able to touch the tub floor with her feet provides her with a sense of security while getting familiar with the floatie. It also gives her the depth needed for her to bounce-walk around the tub as if your baby’s walking on the moon.
However, as your baby becomes more active, stronger and bigger, it becomes very important that there is enough water so that when they kick off the bottom, they can’t kick too hard. Doing so might cause your baby to flip over, and repetitive forceful pushing against the tub wall can wear down the floatie. You can prevent too forceful a kick by making sure that your baby’s feet only barely touch the floor when their legs are straight.