Hello Lakeside,
I read through hundreds of posts on this site during my tx, and no one mentioned swimming that I remember, but I could have missed it. Most of them mentioned waiting X number of hours after applying the treatment. In my case I usually waited 5 hours after tx, before showering, washing, or vasolining, etc. I think the magic number of hours was 3, but some mentioned 2 hour waits. This is to let the tx work into the skin. Some call it binding with the skin.
Everyone is different, but I am pretty sure that even if your dermotologist says you can, you will not want to swim much from about day 12 of treatment to about 5 days after your treatment ends. Unfortunately, it will probably hurt too much. During those days of my tx, I was able to take showers, but could not let the water hit my face for too long, and usually let it angle off of my hands first to slow the force of the water. When my face dried, after I showered, it was extremely painful, 10 out of 10 pain, which would subside immediately after I applied Vasoline or Biafine (doctor prescribed.) The reason was that the shower would cause the loose skin to come off, maybe a little too fast and expose the new skin. Maybe you could figure out a way to lather up with vasoline and swim then. I would call and ask the Dr. if this is OK, then get as much swimming in as you can before it gets too painful.
I got most of my information from this one thread on this site. You can spend hours reading this one thread. http://www.cancercompass.com/message-board/message/all,4537, you click on the LAST button it will take you to the last post which was this week.