sorry to hear you're having sciatica pain. likely this is the result of improper stabilization of the back, and particularly of the lumbar spine. without stabilization particularly there, you will continue to twist that area, causing disc issues which result in sciatica pain.
i would go for a posture first solution. if you can find a good posture expert in your area, i would work with them to get your posture correct. good resources are Gokhale Method http://www.gokhalemethod.com
or Foundation Training http://www.foundationtraining.com
then you will need to learn how to stabilize the lumbar area. while some will do that with tightening up the torso, the preferred method is via generating intra-abdominal pressure (IAP). if you are wondering how to do that, i put up a series of videos on first correcting your breathing to abdominal breathing, and then how to use abdominal breathing to generate IAP.
note that you can abdominal breathe without generating much IAP, so once you get the first 3 drills mastered, i would move quickly to the 4th which is a basic IAP exercise.
then once you do that, i would recommend training how to hold IAP while swimming. the easiest way to do that is using SG and Skate. hopefully you know how to do those.
basically, before you launch off into SG, take an inbreath and generate IAP. hold it, and launch into SG. were you able to hold the IAP or did you lose it? most likely you lost it. keep trying until you figure out how to hold IAP while performing a movement, in this case SG.
once you can do that with SG, then move to SG->Skate. could you hold IAP now in a rotational situation? repeat until done.
then try doing it via swimming.