I remember when going through scuba diving certification, the instructor mentioned that smokers generally get more time out of a tank because their lungs absorb oxygen more efficiently or something like that. And she smoked a lot.

There is no advantage to smoking as a diver. Gas consumption rate in scuba diving has nothing to do with lung absorption capacity (unless you have some kind of serious medical condition). We're usually not exerting at anywhere near VO2 Max. The urge to breathe faster is driven mainly by stress and CO2 levels. New divers are often nervous and flailing around due to having poor trim and buoyancy control. Experienced instructors don't have that problem regardless of smoking status.

Smoking does reduce appetite and therefore can help in weight loss/getting in shape. (I don’t smoke)

I have definitely seen more than a few smokers who quit gain weight, no idea if it's picking up a new addiction or if the smoking subdued their hunger pangs, or some combination thereof.

Nicotine is absolutely scientifically known to be an appetite suppressant.

Getting rid of all the tar and chemicals takes a significant daily toll on the body, which has a definite caloric cost.

That sounds like the sort of stuff that is completely correct. Low fitness is significantly more risky than smoking. (I am not suggesting that anyone start or continue smoking.)

https://peterattiamd.com/all-things-vo2-max/

It's pretty funny seeing people today seethe about the obvious (being unfit is terrible for you) the same way people in the 1970s were seething about the obvious (smoking is bad for you).

There is a reason why life expectancy was dropping in the US years before covid hit.

What obvious-to-others things are you surprised by?

I asked my parents about the "when was smoking viewed as bad for you" bit. They said late 1980s to early 1990s. They said no one thought it was bad for you in the 1970s -- maybe a stinky habit, that is it.

I grew up in the 70s, and that’s hilariously wrong. Cigarettes had warning labels in the 60s, and cigarettes were called “coffin nails” long before that.

There’s a line in a film from 1983 called The Outsiders which is like “give me one of those cancer sticks”, so I think smoking was understood to be harmful earlier than that.

Nicotine suppresses appetite and is also a potent nootropic. Consuming nicotine in losenge or gum form doesn't have any of the ill effects of smoking or vaping. It's also virtually impossible for a nicotine naive person to become addicted through losenges.

It's still addictive and likely to be horrible for your heart (smoke causes cancer, but nicotine even in low doses causes what looks like heart issues, though unlike cancer these might be reversible).

always wondered what the effects on the gums would be since the saliva concentration would be higher with gums/lozenges and it’s primarily surface absorption vs. airborne nicotine.

Absolutely true, I found it amusing because I came to strongly believe over the years that lack of physical activity is one of the worst things one can do for one’s overall wellbeing. We evolved to be physically active, sitting around all day can be compared to being malnourished or not getting enough sunlight or social contact, etc.