I have lower back issues, dating back to my mid-20's (I'm now 70).
In addition to doing 100 crunch sit-ups everyday, plus some related stretches, I always wear a support belt when I am doing anything strenuous around the house, such as mowing or gardening.
For diving I have made a couple of accommodations.
First, since I wear a weight belt, I have added two trim pockets to my BCD tank strap, and have moved weight up off my lower back, as well as improving my profile in the water column.
Second, I have moved up to a 5mm full wet suit (specifically a BARE Velocity). I find that the thicker neoprene helps my lower back and provide a portion of the support that the belt I wear on the surface does.
I had previously considered using a support belt under my prior 3mm wetsuit, but thought the trim pocket/thicker wet suit approach addressed the issue more appropriately for diving.