When the weight of the object is compared with that of water, if the object is heavier than water, it will sag in water, and if the object is lighter than water, it will sag in water. However, when comparing weights, they must be compared in the same volume.
To explain this simply, for example, when compared to the weight of water in a container of exactly the same size and shape as the stone, if the stone is heavier than the water, it will sink in the water, but if it is a pumice stone, etc., and lighter than the weight of the water, it will sink in the water.
If we compare the human body to water in this way, it is actually only a little heavier than water. Therefore, if you do not move your body in water, you will sink, but if you move your arms and legs to kick water out of your body, or if you fill your lungs with air, you can stay afloat.
However, babies are a little different from adults. When they are babies, they have a lot of fat, which makes them a little lighter than water, so they can stay still and float.