[serkandurusoy]

Fat can naturally float on water due to its density being less than that of water.

The more fat one has, the less overall average density becomes, making it harder to sink. I have a close friend who is very overwheight and she can practically sit upright floating. I must admit I'd usually envy her when I struggled staying afloat while she enjoye a cool refreshment like it were her couch :)

So in short, less fat, more muscle mass, more bone mass harder to stay afloat.

Also the diving suite (which is made from a foam-like material) and some of the hallow diving gear make it harder to sink, hence the need for weights.