Well, I'm really going to shorten this post as I lost the first one I typed. Based on what I read and what I remember I typed earlier:
1. Swimming great for fitness which may improve SAC. My buddy of 12 years used up air quickly when we first started diving together 12 years ago. He got a treadmill somewhere along the line and still uses it religiously - his air consumption has improved immensely. IMO, it is a fitness thing and swimming is great for that.
2. Liked the comparison of
@lowwall post, though the exhalation of air while a swimmer's face is in the water is so that the swimmer doesn't have to make a big blow at the end of the exhalation to make room for fresh air and so that the lungs are empty leaving room for the new air.
3. Whether short or long distances swimmers, the number of breaths are not the most important factor in becoming a better swimmer. We see competitive swimmers taking a certain number of breaths in the event they swim and they do practice that, but most competitive swimmers in a practice, because of the distances they swim and muscles needing oxygen, breathe every 1-2 arm cycles. Consider Michael Phelps swam up to 8 miles a day when he competed!
4. Swimming laps in a pool and exhaling underwater, through however many strokes one wants to take before inhaling, seems like it should carryover to allowing one to dive longer with longer exhalations. Think it would help with buoyancy also. It's easy to practice and concentrate on breathing while in a pool doing laps - what else is there to do

. But even with that practice, once a diver goes underwater, concentration begins to spread out to things like, clearing ears, descent/ascent speed, buoyancy control, getting camera ready, looking for buddy or DM, worrying about a swim through, etc... Oh yeah, I need to concentrate on long exhalations!! I would like to think as
@FishWatcher747 does that maybe some of that can become intrinsic to diving.
5. My opinion: Breathing is breathing, swimming is swimming, and you do both in diving, just differently.
It may not make one a faster swimmer
I laughed at this. I used to tell my swimmers, the one way to swim fast is to swim fast.