When I’m swimming free-style laps my lower back hurts. To counter, I use a float between my legs hence no exercise for the lower body. Is it simply a matter of possibly weak abs/obliques?
Thanks for your question! I am assuming that when you use the float you don't get the back pain. The big difference when you use the float is that you are not kicking. Kicking involves moving your hips into flexion and extension. My suspicion is that you have tight hip flexors which are very common especially in people who sit a fair amount. If the hip flexors are tight your spine will try to accommodate which may lead to the pain you are experiencing.
There are other possible reasons for the problem as well, including poor core strength. A good physiotherapist will be able to assess these problems and set you on the right course to correct them.