I am so sorry Deege - what lousy 'friends' they were......are there any special places to swim that cater for people with chest problems?
We are lucky in as much that a local pool here lets us go there every Thursday afternoon. It is great because everyone helps each other, and is understanding about problems we have. Failing that, go along to a pool independently and just keep up with swimming....however gentle, the exercise will help you.
You are certainly right to vent off about them - and certainly not feeling sorry for yourself!
Good luck....