Barring really traditional (and now very rare) pearl/scallop divers, professional divers aren't doing it by holding their breath.

I never got "deep" into it but everybody told me that breath holding diving can be really dangerous.

I know part of the SCUBA story is that phenomena like nitrogen narcosis are particularly dangerous because you need your cognitive capacity to survive in the underwater environment.

In the surface world I can go to a party and drink eight beers and maybe throw up and act like a dumbass and embarrass myself and then wake up with a headache the next morning. That level of incapacitation under water would likely be fatal.

I think that this way of thinking is a little reductive. Every sport depends on certain intelligence metrics, and the brain is ultimately the operator behind all movement. The intelligence required to read a defense and solve a complex math problem may be different, but being good at either require intelligence.

A professional athlete in team based sports, at any given moment, is parsing a ton of data and responsing with quick reflexes and intuition to their changing environment. For example, quarterbacks in the NFL are reading a defense, parsing coverage, and making split second decisions after the play begins to develop.

A soccer goalkeeper is ensuring precise geometry to stay in an optimal position to make a stop, ensuring they are creating a triangle between the ball and the goalposts to optimize their position relative to the possible shooter.

Ontop of all of the in-game aspects, there is intelligence required to train to optimal levels, and hand waving this away as the coaches responsibility is not based in reality. Professional athletes have to stay very mentally focused in their training off the field to achieve their on the field results.

A lot of people judge professional athletes intelligence based on their communications with reporters and on field interviews, but public speaking ability and intelligence are not necessarily correlated. Your smartest engineer is probably not great at making keynote speeches, and likewise would be particularly terrible if they were making them after exerting extreme effort (like athletes do in post game interviews) or while they are pumped with adrenaline with an elevated heart rate (conditions sideline interviews tend to take place in).

All of this is to say, professional athletes arent all meat heads like most computer programmers and bookworms tend to believe. Your judgement that they aren't smart is probably based off of your bias and you are likely overweighting your analysis on a few notable dumb athletes against the crop.

Also, to top it all off, every sport is different, so you can't lump professional athletes into a single bucket.

Why would you say that? Personally I would say that those who make unwarranted assumptions and post them online are less intelligent than average.