Mermaid has it right. What is trained for as a child develops the body. Butterfly is the perfect example. It is this training that enables the swimmer to be better than the non swimmer as they commence freediving (especially monofin dynamics) in later years.
The principle being the swimmer has trained to be better than the non swimmer, and its the training that counts in their performance. Their body will also have developed the traits that Dave suggests is down to genetics. They will have developed the other specialities of breath control, lung capacity, training disciplines that any other newbie freediver will lack. They may well have one or two tricks to learn (I too always swam down the line and rarely managed to glide).
Now, if the swimmer also happens to have good genetics (maybe 6 foot 6 inches tall), so much the better. If 7 foot tall, better still if they prefer high jump. If smaller, well, they probably wont be Olympic medalists. Because genetics will come into play then. But as a child, nobody can forsee the final body shape.
Dave cant be an olympic weight lifter, because he turned out to be the wrong shape in adulthood. But had he trained to become one, from childhood, he would (regardless of genetics) been a great weight lifter today.
The original question could be asked again . eg Is there a sport that would offer the best foundation to athletes if they come into freediving at around age 25?
So, lets say a swimmer, a rower, a boxer, a dart player, an archer or a person who has never done a sport, a chess player, a flower arranger, a doctor, a scientist, a school teacher, an office worker?
Now, genetics will give a freediving advantage or not randomly to all of the above. But do you think a perfectly genetically endowed flower arranger or chess player will have an advantage over the genetically average but trained swimmer, as they become interested in taking up freediving?
I think training is key. Romanticism (another word for it could be attitude) is key. Belief, enthusiasm, vitality, integrity, passion, endeavour, commitment etc etc , are key. Genetics? Yes, stick that word in the list somewhere too. Where would you stick it? For me, not too near the top of the list.
Freediving for me is like a teenage love, enjoyed for a season, but sadly now just a passing fancy that I would love to revisit. Being genetically poor has never spoiled the enjoyment and rewards that freediving offered me. I brought to the table, what I brought to the table.
I have seen lovely things, and felt closer to things that mattered. I have enjoyed experiences that enriched my life. I have competed with myself and my friends. I have gazed down a line disapearing into the blue and felt to go deeper. I have spoken with my soul.
What could good genetics have given me that I have not already received? A gold medal I suppose.