I like the aerobic benefit of swimming. Like cycling, there is a sweetspot at which one can just keep going and going. Swimming is easy on the joints, and works muscle groups that aren't used in cycling. By mixing up strokes, it's a good way to get a full-body workout.

To answer OP's question about whether swimming can simulate training at high altitude: no. Unless you're swimming in Denver. And then that wouldn't be a simulation.

Regarding getting the HR up and not being able to breathe in a pool.... If you are using good form when you swim, you can always breathe in a pool.

But I will agree that the water pressure on the torso might require a bit more effort, though subconscious, to pull in air. For instance, while treading water, one can feel the difference between drawing a deep breath while in the pool vs doing same standing beside the pool.

That being said, perhaps one could assert that swimming can improve aerobic performance since it requires a bit more effort from the diaphragm, intercostals, and accessory muscles, making them stronger for non-submersive athletic activity.

Any physiologists out there care to comment?