While I am not a smoker, this is what I can offer on the subject.
I've heard from smokers that smoking doesn't affect their diving, except if they have a cigarette directly before a dive - then the negative effect is pronounced. But after 30mins or so of diving they find their lung function normalising again.
I've read studies that smoking doesn't affect your static times at all, even some anecdotal evidence that it might increase your static times slightly.
When it comes to how it affects someone say on a shore dive, having to gun it through the backline, swim against currents, dive for hours, etc., we can look to general knowledge about how smoking affects anyone's ability to maintain levels with any form of higher intensity exercise (i.e. obviously not as good as a non-smoker). Also, recovery times are reduced.
When it comes to lung capacity, here's a quote I found on the web from a surgeon: "I can tell you that there is a tremendous difference in size, elasticity, color, circulation....it is the difference between holding a very squishy stretchy pink structure and a smaller, rubbery somewhat rigid grayish one." I think that says it all in terms of whether smoking limits your potential with regards to lung capacity.
I don't have much info about how it affects the dive reflex, blood gases, etc, so can't offer any comment on smoking vs. deep diving.
My personal opinion is that if you aren't out to break depth or bottom time records, just want to enjoy your spearfishing socially, and have a smoke every now and then, I don't think it's going to impact your diving much at all.
Still, every time I see a diver come back to the beach and light up while still in his suit, it still gives me a little shock - it just looks so wrong to me. But that's just me

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