Redbank,

I'm not an expert by any means, those more experienced will chime in...but don't think of the aeration as "directly" putting oxygen in the water right at the diffuser - it's just pushing water off the bottom to the top, where it gets oxygenated and cycled.

Generally you are going to want to have your diffuser at the deepest location in the pond - that is where it will do the most good.

In regards to swimming...it probably won't make a tremendous difference (depending maybe on how deep it is?) but it's not going to hurt.

Straw