I hope that the steel boat dock is Galvanized.
Aside from that, generally, when attaching wood/composite finish stock/framing/deck boards - to a steel frame, the process is to install (solidly through mechanical fastening) a wood sub-frame at 16" OC. - to the Steel frame.
In this case, you would use Pressure Treated stock for the subframe material.
Then, the finish wood/composite product/deck boards - are attached to that, using conventional SS deck screws.
Also: You need Stainless Steel fine-thread fasteners/lags/threaded bolts - to attach the PT wood subframe to the galv. steel framing.