I had sciatica problems for the last few years. While everybody told me swimming is good for your back, I experienced otherwise.
Especially with backstroke, which is what everybody recommends for back problems.
Breakstroke, which was the one style that I was forbidden, was the best for me... żżż???
Then a little longer than a year ago I had a lumbar operation. Since then everything seems to go back to normal. Swimming is a sport, a funny time and a therapy for me (both mental and physical).
Stretching my back muscles is necessary and I floow TI style 100% (or so I believe).
Focus on stretching your arms, dorsal muscles, rotate a little bit just to use your side muscles when pulling back and try to lelax your back, supporting no weight at all.
Also I strech my back and legs before entering the water (well, I do that exercise quite a few times during the day).
I place my arms above my head. All along to the roof (or sky).
I relax my lumbar area.
After a while, when I am relaxed and feel my breathing with my belly, I take a bow and try to touch my feet with my hands. Usually, especially in the morning, it is difficult. So I just move my hands up and down my legs, very carefully, touching ankles and feet.
The point is to relax your muscles and fascia, especially. Once you feel relaxed, you need to compensate your legs. Front and back muscles should maintain the same amount of weight.
Once I have done this, it is the time to go up. Your butt should be way behind your feet/ankles and i go up slowly.
Once you are standing up, it is time to repeat all the process. The second time, when down, use that time to breath using your belly. Deeply. That will stretch your psoas muscles without doing dangerous exercises. At least five times you should inhale and exhale, relaxed.
Then go up again and feel purpously how well you are feeling and, if you are there for a swim, feel how well you are going to swim, how effortless you are going to do it.
If I have time enough, or feel like it, I do a third repetition, then using my arms when I am down, placing them over my shoulders and neck, stretching my high-back muscles.