I didn't touch on the things that didn't help on here but over the course of the last 6 or more years, planks (without the balance exercises) aggravated my situation. From the majority of my reading, weak glutes cause a bunch of issues with the back. I found a video on youtube from Athlean-X about lower back spasms that got me kick started and started giving me some relief. From there, once I learned a little more, I found a handful of exercises that work on the glutes and have had more recovery than I thought I was ever going to see again. Last August through October, I thought I might be done skiing forever,
by the end of November, I was back skiing once a week and have picked it up to 3 times a week now. I get a little stiff here and there but for the most part, very few spasms (and the ones I do have, i can work out the glute stuff and it tends to fix them fairly quickly).
I didn't want to discourage the planks because I think they will benefit a lot of people (me included), but if you have a desk job, the glute work will help you. For the record, my back spasms were not just lower back, they seemed to migrate to various muscles throughout the back. However, it was when the lower back got really bad that I had to shut down skiing completely.
@ScottScott had a much better description of the deep water start that I employ.