Main issue with swimming is the heavy chlorine exposure you’d get from doing it often. Otherwise it should be the healthiest.
IIRC, the Olympic “swimmer’s body” is considered to be the healthiest possible shape and proportion a human body could have, scientifically speaking.
But swimming DOES have impact, if one doesn’t have the proper technique you can get loads of problems from your shoulders, back, elbows. Even with good technique you can be prone to injuries that can keep you out for weeks or years. I know a few people on my adult team who have been out of the water for a few years because of shoulder problems.
There are the occasional drownings, even with adult swimmers, though a lot happen when someone is screwing around, or in open water if a current gets bad.
Yes swimming is good, especially if one doesn’t over do it and listens when something hurts.
That would be unlikely if, as someone mentioned above, tennis players seem to have the longest longevity.
And, as also mentioned, there’s the potential chlorine issue.
There are probably too many activities like walking, running, jogging, (common) dancing, etc. that are a bit too “leg” focused. Swimming is unique in being closer to a full body activity while having the sort of zone-out monotony that attracts people who like walking, running, etc. But that doesn’t mean that you can’t achieve similar results by mixing walking, climbing, Olympic lifting, rowing, dancing, etc.
Probably the best activity is the one that you’ll actually do.
Golf was the one I immediately considered since golf is routinely played between people of different skill levels, and an older golfer can compete with younger more capable golfers, by applying their handicap. This put the activity safely into the “sport” category rather than “fitness activity”.
Swimming, it appears you can do a similar thing with handicap races. If your natural time is 10 seconds longer than a competitor, you get to start 10 seconds earlier. You don’t have to push your body to its limits in order to participate in a swim competition, making the sport safer and healthier.
Me. I have arthritis in my knees.
On a similar note, crew (rowing) is very likely to lead to scholarships, and education sets you up well for later in life.
I imagine that firearms sports are probably pretty close to neutral on the body: Not much physical activity, beneficial or stressful.
For swimming, I look at Michael Phelps’ typical daily Calorie intake and shudder. I have a hard time believing that it’s possible to maintain a healthy nutritional balance that way, and after you retire from swimming, it’s going to be very difficult to transition to a normal diet.
There is a difference between playing a sport at the elite professional level and being proficient at it at the regional level.
Long distances or large weights place a lot of stress on joints and connective tissues. Sports involving many repetitive motions do too. Sports differ in injury risk, but minor strains are not the same as lifelong brain or joint damage. Some exercise helps reduce injury and arthritis as does building balanced muscle strength.
Exercise is healthy; socializing is healthy; a mixture of strength and cardio is ideal. You also want a sport one can easily practice for decades - making bobsledding impractical.
I’m sure there are many sports which are safe and healthy. I would guess few studies compare sports but things like tennis are accessible, social, may benefit from “upper class” correlations, can be scaled to level and mainly stress the shoulder joints. Not a bad choice.
Short and middle distance swimming probably offers a lot of benefits. Probably the same is true for similar races. Longer distances probably are more stressful with less health benefit. Not all combat sports are contact, so some of the martial arts might be healthier than you believe.
I’m guessing that Chess Boxing is right out.
I meant impact in the literal sense of your feet hitting the ground. That is what caused the long term knee damage for runners
I’d argue that if a sport requires enough moving around to actually raise your heart rate and significantly make a positive impact on your health, then there’s going to be some injuries like this associated with it. But by avoid any kind of impact (in the literal sense) swimming must surely be the most healthy.
Though I guess the counter argument is, if the alternative is sitting on the couch watching TV, sports like golf or pool probably have the greatest net increase in health versus injuries, even if they never raise your heart rate that much. Though you should take into account the increased booze consumption associated with those sports, plus the chances of getting stabbed or hit by lightning 
I’m not sure about swimming but I know that in strongman and bodybuilding that the frequency and scale of eating is so laborious that it’s really not that hard to go back to something more normal.
To be sure, some people just love food and calories and, for them, a lifetime of training like an athlete might be their best option (or GLP-1 agonists). But that’s not most people.
I don’t think “sport” necessarily means “competitive sport”.
I think walking or hiking probably has to be the healthiest. Our bodies are evolved to excel at that. Though, maybe the evolution is at least partly for long distance running, and I don’t think that’s the healthiest.
It is not at all uncommon for pro golfers to experience spine and joint stress injuries just from the 100s of 1000s (millions?) of balls they hit regularly. I suspect you are falling prey to that fallacy where you think of the folk you have heard of who DID NOT experience injuries and enjoyed long careers.
Look instead for players who seemed really big for a while - and then either receded or fell off the map. It is a good bet that at least a few of them did so due to injuries. Just off the top of my head, Michelle Wie, Natalie Gulbis … Freddie Couples was notorious for his back injuries. Even a guy named Tiger had one or 2 surgeries before driving off the road.
I’ve golfed for 50+ years. I enjoy it, and it is more exercise than sitting in a chair, but I’m not sure I’d say I experience significant health benefits from golfing.
I swim and bike - and would think of those as conveying far more health benefits. Never injured doing either except the rare occasions that I fell off my bike or got hit by a car.
More normal, sure. But I’d expect that the new equilibrium would still be rather more than healthy.
Pro golfers create a level of torque with their bodies that just doesn’t occur with weekend golfers. As mentioned, Tiger was a prominent example of someone whose back couldn’t routinely sustain what he asked of it. Same with Fred Couples. That’s not to say the repetition isn’t a factor.
It’s awesome to drive a ball 350 yards, I’m sure. But there’s a price, and the typical weekend golfer doesn’t have nearly the same risk.
Yeah - I got somewhat confused as to what level athletes we were discussing, as some folk were talking about O-level swimmers.
I still don’t think golf is much exercise. A good brisk walk followed by a bit of stretching is just about as good.
Yeah I’m gonna be that person: cite? Swimmers’ bodies are most the result of selection for the sport. Tall lean long limbed. Which actually is associated with shorter life expectancy than being short.
I’ll call BS here too. Regular running at recreational levels anyway actually protects the knees from damage.
The variety argument would suggest triathloning but there is a real risk of death in the open water swim. And there is some selection to compulsive overtraining in the athletes.
The main study supporting the tennis claim: