You know, swimming was also considered an essential skill of the Japanese samurai and was aproached with the same energy as the more martial aspects of their training. Ninja in particular were known for swimming skills including apnea swimming if it got them to their goals.
Personally, I think that it is natural to find comparisons between swimming skills and martial arts, or more accurately, Asian religious practices.
I also think that after spending a decade and a half as a youngster and a young adult going through swimming training as rigorous as any martial arts course - 3 hours in the am from 4 to 7:30 and 3 hours in the pm from 3:30 to 6:30 plus stretching and wieght training. 4500yds or more per hour.... hypoxic training, IM and stroke training, turns and starts, Olympic coaches and serious competition....it is natural that such immersion in an effort would lead to religious perspectives.
I also did martial arts of various kinds as a kid and a young adult and the parallels were always present. The physical training from a rigorous 11 month competitive swimming club made me a match for most other atheletes including a lot of sparring partners. I did learn about calming down, containing my emotions and focusing on the target but the fact is, swimming had already taught me all of that. I fought at tournements, often in open divisions, and did quite well for myself but most of the ethical and internal strength to compete came from swimming training and competition rather than martial arts which, instead, served to reinforce those understandings. Swimming was the core of my martial arts ability, not the other way 'round.
My point is that it isn't necessary to graft freediving onto some outside meditative practice. The art itself is teaching us how to do it, how to get better at it, how to avoid the dangers while expanding the experience, how to deal with our minds and our bodies.... the undertaking of Freediving itself is a path with its own rules and its own requirements. While it might work well to hybridize certain breathing patterns like Ibukai or TanTien techniques from various systems of fighting and meditation, it may work even better to let the art of Freediving itself teach us what it will about physiology, psychology and metaphysics. We will create specific breathing patterns, meditations, spiritual understandings, etc. If we develop as a community, and we are, that will already be in play as I write this
The experience of sliding off a small boat, miles from the big boat, into very deep blue water with no one around and being left to one's own devices is big enough and challenging enough to equal any staredown with some foul breathed, scruffy looking, pugilistic malcontent. You will learn as much or more about yourself trying to identify a large shadow in blue water as you will looking over cold steel...
There are as many paths to enlightenment as there are people to walk them, and I think Freediving can be a path to enlightenment all by itself. From it one can learn about chi and about breathing, about stilling the mind and listening, about integrating with one's environment, about compassion and humanity....there are no differences between this and any other bona fide Way.