The surfski paddle stroke is essentially the same as the sprint kayak stroke (let's ignore variations you may make in technique on rough water). Two of the most common health issues that sprint kayaking programs report relate to shoulders and lower backs. Issues with other joints probably tend to become more common as paddlers age.
A good, powerful stroke includes significant torso rotation. Torso rotation needs to be accompanied by hip rotation. If the hips do not rotate in concert with the torso then significant stress can be concentrated in the lower back and likely contribute to lower back strain or injury. Emphasis on leg drive should also include parallel hip rotation. Avoiding inadequate hip rotation can sometimes be as simple as making sure your hips and seat are as friction free as possilble (though maybe not so far as to make you slide out of your seat too easily)
And don't neglect the basic stretches that are recommended for lower back health.
Lower back pain: causes, treatment, and prevention of lower back injuries and pain - Peter Attila conversation with Stuart McGill
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Stuart McGill's big 3 daily exercises for the back
Don't do any that cause significant discomfort. Maintain good form and do to the extent you can (if you can maintain balance and control without strain). Stick with the ones that do feel balanced, controlled and not strained.