This depends on your baby’s height. The first time or two, fill the bathtub with enough water so that her feet barely touch the bottom of the tub when she is 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 she’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 she kicks off the bottom, she does not kick too hard. Doing so might cause her to flip over or repetitive forceful pushing against the tub wall can wear down the floatie. You can prevent too forceful a kick by making sure that her feet only barely touch the floor when her legs are straight.