We use a single cross brace on each section of dock (see image 1) - which might keep your costs down - we we find to be quite effective. You'd want to alternate diagonal directions on each section. These get tucked up under the dock as high as they can go to be most effective; in fact, the final brace usually is installed above the dock platform until the water warms sufficiently and we go swimming to move it to the underside.
[Image credit docks.com]
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