This was a fun question and you probably expect a smart remark, but my friends, this is science. When a person floats or sinks is dependent of how much water they displace in relationship to their own body weight. If a person weighing 300 lbs displaces 310 lbs of water then they will float. If the 300 lb person only displaces 290 lbs of water, then they will sink.
The explanation of buoyancy was expressed around 287 BC by a gentleman by the name of Archimedes who was a noted astronomer, inventor, physicist, mathematician and engineer. The explanation has endured where today it is know as the “Archimedes Principle” on buoyancy. I wrote an article that is easy to understand on Archimedes Principle.