@Mazda, I have the same reflex setup. While I don’t think I felt worse back pain after switching, I think the above comments likely hold your solution. I too only ski in certain bathing suits with low resistance (don’t catch water in the legs), press knees tight together (less resistance on legs), and use that core, not just the low back.
In terms of exercises, Google “McGill Big 3”. Developed by researcher and doctor Stuart McGill up in Canada. I’ve read multiple testimonials from people who say if you do at least these 3 exercises (curl ups, side planks, bird-dog) EVERY day (it’s about 20min) your core will be in much better shape. To @9400 point, I’ve added gute bridges to my daily routine as well (again too much desk sitting) - big difference! I’ve been doing these 4 exercises for a month and have never felt stronger!
One interesting thing - through his research he says planks held more than 10sec aren’t helpful. Better to do multiple reps of 10sec holds.