One of my biggest regrets from college was selling the book we used in my philosophy of sport and play class. There was an entire chapter dedicated to defining why scuba diving, sky diving, auto racing, mountain climbing, etc., are sports and was incredibly well-written, categorized and presented. I could do a much better job by quoting the thinkers who defined the philosophy of sport, play and games with that resource.
However, I'll do my best to remember some of the arguments regarding this topic and present them so we can better understand diving's right to be a sport under the traditional classification of a sport in ancient Greece.
If we look at how a modern sport is defined, we could use the following:
DEFINITION OF SPORT:
Sport Definition | Definition of Sport at Dictionary.com
1) Physical activity that is governed by a set of rules or customs and
often engaged in competitively.
2) A particular form of this activity.
3) An activity involving physical exertion and skill that is governed by a
set of rules or customs and often undertaken competitively.
4) An active pastime; recreation.
For the sake of argument, and I might agree with Halemano, let's toss out number 4 as being "watered down" and we'll look at definitions 1, 2 and 3 as they apply to scuba diving. The first 3 definitions, I think we could agree defines most generally accepted definitioms for a sport and was also how the Greeks separated a sport, from a game, and from play. Games and play do not need physical activity to the same level as a sport. For example, chess is a game that only requires the player to move a playing piece and may be played without the player moving his own pieces. Play, such as gazing at clouds and pretending they are other shapes, requires very little physical activity and also has little if any rules or defined goals.
Scuba Diving
Physical Activity: Divers must swim or use propulsion vehicles. Swimming and operating DPV's both require skill. Swimming may require exertion in current or when needing speed. Scootering may require skills similar to that of race car drivers. As such diving can be 100% physical or based upon the skill of operating and using one's body in conjunction with the operation of a machine. Even slow swimming requires attention to good propulsion and awareness of buoyancy and trim to meet some of the goals of diving such as protecting the environment.
Rules: Ascent rates, deco stops, gas management, dive protocols, automatic behavioral responses to prevent injury or death, safe use of equipment such as reels, scooters and spearguns, team and solo procedures, practices for certain types of diving such as wreck, ice, cave, rebreathers, freediving, and underwater hunting.
Competition: The competition is man vs. nature and the physical laws of the universe in the purist sense. The goal is to win by surviving the dive.
Forms: The most obvious competitive forms of diving often involve freediving. Underwater hockey, underwater rugby, static apnea, dynamic apnea and depth competitions as well as spearfishing competitions are often used to defend diving as a sport. But, other forms of diving are also competitive such "monkey bash" with scooters or the games that are part of the Ultimate Diver Challenge. Notice I said games. You can play a game in a sport, but not the other way around. Whatever the form of diving whether cave diving, wreck diving, nitrox, trimix, reef or other form, the competition exists within oneself or with the environment.
Goals: The ultimate goal is to survive. Since survival is a goal, there must be forces that need to be overcome and beaten. These may be dangerous marine life, the ocean's currents and tides, the geology of a cave or the bottom features of a reef. Secondary goals might be to take photos, protect the environment through swimming skills, hunt, explore, discover, etc.
Ultimately, any sport, game or sense of play has to be "free" of our daily lives and "unreal" in the sense that the rules and landscape of that sport, play or game exists with its own rules and time frame, and finally engaged in for fun.
All play is free and unreal and therefore, recreation.
Things get more complicated when we attempt to discern whether professional athletes are "playing" a sport. But, the answer is that they are ultimately playing since they are still obeying the rules and time frame of a world that can be distinguished from daily life by a different set of rules and time frame.
It's been 20 years since I've studied the nuances of this topic. I've done my best to recollect the arguments supporting diving as a sport. It is classified as a sport of vertigo like skydiving, stunt flying or gymnastics.