And swimmers have lower mortality rates than walkers and runners.
"After adjustment for age, body-mass index, smoking status, alcohol intake, and family history of cardiovascular disease, swimmers had 53%, 50%, and 49% lower all-cause mortality risks than did men who were sedentary, walkers, or runners, respectively (p < 0.05 for each)."
As a runner and cyclist, in terms of difficulty swimming is in another dimension. It requires all four limbs to act in coordination with breathing, it has higher temperature stresses, and if you get tired you drown. The average swimming duration of a triathlon is less than a fourth of the others.
"After adjustment for age, body-mass index, smoking status, alcohol intake, and family history of cardiovascular disease, swimmers had 53%, 50%, and 49% lower all-cause mortality risks than did men who were sedentary, walkers, or runners, respectively (p < 0.05 for each)."
https://scholarworks.bgsu.edu/ijare/vol2/iss3/3/