The first thing you need to do is get the right gear: comfortable swimwear, good fitting goggles, swim fins, swim paddles and/or aquatic gloves well adjusted to your size, an elastic and possibly a training board and a pull buoy (these last two items you can easily find by the swimming pools).
For this swimming material, ask a professional for assistance on what to get.
In so far as swimming is a sport in which your body is carried, you can practice it as soon as you're done with your perineal re-education.
Start practising this sport as soon as your doctor allows it.
That way you will gently start working out again.
Take advantage of these sessions to relax while solliciting your muscles and toning your body.
As with all physical activities, small warm up exercises are recommended before you start training and stretching exercices are recommended after you're done training.
Don't forget to drink water on a regular basis in order to optimize your recovery.
Breathing is very important when swimming and depends on the pace at which you're swimming;
Make sure you breath out (exhale) deeply and completely in the water, before breathing in (inhale), so as to optimize your comfort and your muscle relaxation.
Don't forget to put a fruit or a cereal bar in your bag in case you get a little hungry
Ideally, choose to workout on Wednesdays or on Saturdays. You may choose to workout on a different day but in that case please make sure you respect the off days between each session.
Good luck and make sure you always enjoy these sessions.